Method and apparatus for performing handover procedure in wireless communication system

ABSTRACT

A method performed by a terminal in a wireless communication system includes: receiving, from a base station, a radio resource control (RRC) reconfiguration message including configuration information for conditional handover; identifying a cell for conditional handover based on the configuration information for conditional handover; and triggering a packet data convergence protocol (PDCP) status report for a radio bearer, according to an execution of a conditional handover for the identified cell for conditional handover.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2019-0051827, filed on May 2, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety.

BACKGROUND 1. Field

The disclosure relates to a method and apparatus for performing a handover procedure in a wireless communication system.

Also, the disclosure relates to a method and apparatus for processing communication data in a wireless communication system.

2. Description of Related Art

To meet the increasing demand for wireless data traffic after the commercialization of 4^(th) Generation (4G) communication systems, efforts to develop enhanced 5^(th) Generation (5G) communication systems or pre-5G communication systems are continuing. For this reason, a 5G communication system or pre-5G communication system is called a Beyond 4G Network communication system or a Post Long Term Evolution (LTE) system.

To achieve a high data transmission rate, the implementation of a 5G communication system in an ultra-high-frequency millimeter wave (mmWave) band (for example, a 60 GHz band) is being considered. To reduce path loss of radio waves and increase a transfer distance of radio waves in an ultra-high-frequency band, in a 5G communication system, technologies for beamforming, massive Multiple Input Multiple Output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, analog beam-forming, and large-scale antenna are being discussed.

Also, to improve the network of the system, various technologies, such as advanced small cell, cloud radio access network (cloud RAN), ultra-dense networks, Device to Device communication (D2D), wireless backhaul, moving networks, cooperative communication, Coordinated Multi-Points (CoMP), and interference cancellation, have been developed for 5G communication systems. In addition, other technologies, such as hybrid frequency shift keying (FSK) and quadrature amplitude modulation (QAM) Modulation (FQAM) and Sliding Window Superposition Coding (SWSC) based on Advanced Coding Modulation (ACM) and Non Orthogonal Multiple Access (NOMA) and Sparse Code Multiple Access (SCMA) based on Filter Bank Multi Carrier (FBMC), have been developed for 5G communication systems.

Meanwhile, the Internet is evolving from a human-centered network in which humans generate and consume information to an Internet of Things (IoT) network in which distributed components such as objects transmit, receive, and process information. Internet of Everything (IoE) technology resulting from combining IoT technology with big data processing technology, etc. through a connection to a cloud server or the like is on the rise. To implement the IoT, various technical factors, such as sensing technology, wired/wireless communication, network infrastructure, service interface technology, and security technology, are required. Recently, technologies including a sensor network, Machine to Machine (M2M), and Machine Type Communication (MTC) for connections between objects have been studied. In an IoT environment, an intelligent Internet Technology (IT) service is provided to collect and analyze data generated by connected objects to create new value for human life. The IoT may be applied to various fields, such as smart homes, smart buildings, smart cities, smart cars/connected cars, smart grids, health care, smart appliances, and advanced medical services, through convergence and combination between existing Information Technology (IT) and various industries.

Accordingly, various attempts to apply a 5G communication system to an IoT network are being made. For example, technologies, such as a sensor network, M2M, and MTC, are implemented by technologies, such as beam forming, MIMO, and array antenna, which are 5G communication technologies. Applying a cloud radio access network (CRAN) as big data processing technology is also an example of the convergence of 5G technology and IoT technology.

With the development of wireless communication systems as described above, various services are being provided. Therefore, a method for stably supporting services related to, particularly, handover of a user equipment (UE) is required.

SUMMARY

The disclosure provides an apparatus and method capable of effectively providing a service in a mobile communication system.

Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments of the disclosure.

According to an embodiment of the disclosure a method performed by a terminal in a wireless communication system includes: receiving, from a base station, a radio resource control (RRC) reconfiguration message including configuration information for conditional handover; identifying a cell for conditional handover based on the configuration information for conditional handover; and triggering a packet data convergence protocol (PDCP) status report for a radio bearer, according to an execution of a conditional handover for the identified cell for conditional handover.

Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely.

Moreover, various functions described below can be implemented or supported by one or more computer programs, each of which is formed from computer readable program code and embodied in a computer readable medium. The terms “application” and “program” refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or a portion thereof adapted for implementation in a suitable computer readable program code. The phrase “computer readable program code” includes any type of computer code, including source code, object code, and executable code. The phrase “computer readable medium” includes any type of medium capable of being accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disc (CD), a digital video disc (DVD), or any other type of memory. A “non-transitory” computer readable medium excludes wired, wireless, optical, or other communication links that transport transitory electrical or other signals. A non-transitory computer readable medium includes media where data can be permanently stored and media where data can be stored and later overwritten, such as a rewritable optical disc or an erasable memory device.

Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1A shows a structure of a long term evolution (LTE) system to which an embodiment of the disclosure may be applied;

FIG. 1B shows a radio protocol architecture of an LTE system to which an embodiment of the disclosure may be applied;

FIG. 1C shows a structure of a next-generation mobile communication system to which an embodiment of the disclosure may be applied;

FIG. 1D shows a radio protocol architecture of a next-generation mobile communication system to which an embodiment of the disclosure may be applied;

FIG. 1E is a view for describing a procedure in which a next generation Node B (gNB) according to an embodiment of the disclosure releases a connection of a user equipment (UE) for the UE to change from a Radio Resource Control (RRC) connected mode to an RRC idle mode and a procedure in which the UE sets up a connection to the gNB to change from the RRC idle mode to the RRC connected mode;

FIG. 1F shows a handover procedure in a next-generation mobile communication system according to an embodiment of the disclosure;

FIG. 1G shows a conditional handover procedure according to an embodiment of the disclosure;

FIG. 1H shows operations of a conditional handover procedure of a UE according to an embodiment of the disclosure;

FIG. 1I shows a structure of a UE or a wireless node according to an embodiment of the disclosure;

FIG. 1J shows a block configuration of a Transmission/Reception Point (TRP) or a wireless node in a wireless communication system according to an embodiment of the disclosure;

FIG. 2A shows a structure of an LTE system to which an embodiment of the disclosure may be applied;

FIG. 2B shows a radio protocol architecture of an LTE system to which an embodiment of the disclosure may be applied;

FIG. 2C shows a structure of a next-generation mobile communication system to which an embodiment of the disclosure may be applied;

FIG. 2D shows a radio protocol architecture of a next-generation mobile communication system to which an embodiment of the disclosure may be applied;

FIG. 2E is a view for describing a procedure in which a gNB according to an embodiment of the disclosure releases a connection of a UE for the UE to change from an RRC connected mode to an RRC idle mode and a procedure in which the UE sets up a connection to the gNB to change from the RRC idle mode to the RRC connected mode;

FIG. 2F shows a handover procedure in a next-generation mobile communication system according to an embodiment of the disclosure;

FIG. 2G shows RLC header formats that may be applied in a 1-1 embodiment which are operations of a reception RLC layer in which a RLC UM mode according to an embodiment of the disclosure is set;

FIG. 2H shows RLC header formats that may be applied in a 1-2 embodiment which are operations of a reception RLC layer in which an RLC AM mode according to an embodiment of the disclosure is set;

FIG. 2IA shows RLC header formats that may be applied in a 2-1 embodiment, and are operations of a reception RLC layer in which an RLC UM mode according to an embodiment of the disclosure is set;

FIG. 2IB shows RLC header formats that may be applied in a 2-1 embodiment, and are operations of a reception RLC layer in which an RLC UM mode according to an embodiment of the disclosure is set;

FIG. 2J shows RLC header formats that may be applied in a 2-2 embodiment which are operations of a reception RLC layer in which an RLC AM mode according to an embodiment of the disclosure is set;

FIG. 2K shows operations of a reception RLC layer of a UE according to an embodiment of the disclosure;

FIG. 2L shows a structure of a UE or a wireless node according to an embodiment of the disclosure; and

FIG. 2M shows a block configuration of a TRP or a wireless node in a wireless communication system according to an embodiment of the disclosure.

DETAILED DESCRIPTION

FIGS. 1A through 2M, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged system or device.

Hereinafter, an operation principle of an embodiment of the disclosure will be described in detail with reference to the accompanying drawings. However, when detailed descriptions about known functions or configurations associated with the disclosure are determined to unnecessarily obscure the gist of the disclosure, the detailed descriptions will be omitted. Although the following terms are defined in consideration of the functions of the disclosure, they may vary according to a user or operator's intentions, judicial precedents, etc. Hence, the terms must be defined based on the contents of the entire specification.

Throughout the disclosure, the expression “at least one of a, b or c” indicates only a, only b, only c, both a and b, both a and c, both b and c, all of a, b, and c, or variations thereof.

Examples of a terminal may include a User Equipment (UE), a Mobile Station (MS), a cellular phone, a smartphone, a computer, a multimedia system capable of performing a communication function, or the like.

In the disclosure, a controller may also be referred to as a processor.

Throughout the specification, a layer (or a layer) may also be referred to as an entity.

When detailed descriptions about known functions or configurations associated with the disclosure are determined to unnecessarily obscure the gist of the disclosure, the detailed descriptions will be omitted. Hereinafter, an embodiment of the disclosure will be described with reference to the accompanying drawings.

In the following descriptions, the terms used to identify connected nodes, the terms indicating network entities, the terms indicating messages, the terms indicating interfaces between network objects, the terms indicating various identification information, etc. are examples for convenience of description. Accordingly, an embodiment of the disclosure is not limited to these terms, and other terms having the equivalent technical meaning may be used.

For convenience of description, an embodiment of the disclosure uses terms and names defined in a 3^(rd) Generation Partnership Project Long Term Evolution (3GPP LTE) standard and/or a 3^(rd) Generation Partnership Project New Radio (3GPP NR) standard. However, an embodiment of the disclosure is not limited to the terms and names, and may be applied in the same way to a system based on another standard. In an embodiment of the disclosure, evolved Node B (eNB) is mixed with next generation Node B (gNB) for convenience of description. That is, a base station mentioned as an eNB may represent a gNB. Also, the term UE may represent other wireless communication devices, as well as mobile phones, NB-IoT devices, and sensors.

A post LTE communication system, that is, a 5^(th) Generation (5G) communication system needs to freely reflect various requirements from users and service providers. Therefore, the 5G communication system needs to support services satisfying various requirements simultaneously. Examples of services considered for the 5G communication system are Enhanced Mobile BroadBand (eMBB), massive Machine Type Communication (mMTC), and Ultra Reliability Low Latency Communication (URLLC).

According to an embodiment of the disclosure, the eMBB is aimed to provide a data rate that is higher than a data rate supported by existing LTE, LTE-A or LTE-Pro. For example, in a 5G communication system, the eMBB needs to provide a peak data rate of 20 Gbps in a downlink and a peak data rate of 10 Gbps in an uplink with respect to a base station. Also, the 5G communication system needs to provide a high user perceived data rate, while providing a peak data rate. To satisfy the requirements, the 5G communication system requires an improvement of various transmission/reception technologies including enhanced Multi Input Multi Output (MIMO) transmission technology. Also, existing LTE transmits signals by using a transmission bandwidth of maximally 20 MHz in a 20 GHz band, whereas the 5G communication system uses a wider frequency bandwidth than 20 MHz in a frequency band of 3 to 6 GHz or more, thereby satisfying a required data rate.

Simultaneously, mMTC is considered to support an application service such as Internet of Thing (IoT) in the 5G communication system. The mMTC may require supporting connections of massive UEs (a large number of UEs) in a cell, UE coverage enhancement, an improved battery lifetime, UE cost reduction, etc. to efficiently provide the IoT. The IoT is connected to various sensors and devices to provide a communication function. Therefore, the IoT needs to support a large number of UEs (for example, 1,000,000 UEs/km²) in a cell. Also, because a UE supporting the mMTC has a high probability of being located in a shadow zone such as the basement of a building, which is not covered by a cell, due to its service characteristics, the UE may require wide coverage compared to those required by other services that a 5G communication system provides. The UE supporting the mMTC needs to be a low-cost UE, and requires a long battery lifetime of about 10 years to 15 years because it has difficulties in frequently replacing the batteries.

Finally, URLLC which is a mission-critical cellular-based wireless communication service may be used in services of remote control for robots or machinery, industrial automation, unmanned aerial vehicle, remote health care, emergency alert, etc. Accordingly, communication provided by the URLLC may need to satisfy ultra-low latency and ultra reliability. For example, a service supporting the URLLC needs to satisfy air interface latency that is shorter than 0.5 milliseconds, and simultaneously, the service may have a requirement of a packet error rate that is equal to or lower than 10⁻⁵. Accordingly, for services supporting the URLLC, the 5G system needs to provide a smaller Transmit Time Interval (TTI) than other services, and simultaneously, the 5G system requires a design of assigning a wide resource in a frequency band to secure reliability of a communication link.

Three services (that is, eMBB, URLLC, and mMTC) which are considered in the above-described 5G communication system may be multiplexed in a system and transmitted. In this case, to satisfy different requirements of the services, the services may use different transmission/reception methods and different transmission/reception parameters. However, the above-described mMTC, URLLC, and eMBB are examples of different services types, and service types to which the disclosure is applied are not limited to the above-described examples.

Also, hereinafter, embodiments of the disclosure are described based on examples of LTE, LTE-A, LTE Pro, or a 5G (or NR, next-generation mobile communication) system, however, the embodiments of the disclosure may be applied to other communication systems having similar technical backgrounds or channel types. Also, the embodiments of the disclosure may be applied to other communication systems through slight modifications within a range that does not greatly deviate from the scope of the disclosure, under a determination of a person having a skilled technical knowledge.

As described below, the disclosure proposes a conditional handover method that is efficient in a next-generation mobile communication system. According to an embodiment of the disclosure, an issue that a source gNB starts late data forwarding to a target gNB and an issue caused when a source gNB fails to recognize a connection release time of a UE may be overcome.

The next-generation mobile communication system may support various handover methods for reducing a data interruption time that may be caused when a handover occurs. Particularly, according to a conditional handover procedure, a source gNB sets a condition for a UE together with a plurality of target gNB candidates, and the UE checks signals from the target gNB candidates based on the condition to select a target gNB candidate from among the plurality of target gNB candidates as a target gNB, thereby starting a handover procedure. Accordingly, because the source gNB is not capable of expecting a target gNB to which the UE will perform a handover, the source gNB starts late data forwarding to the target gNB, and also the source gNB may not recognize a time at which the UE releases a connection to the source gNB.

Hereinafter, a method and apparatus for performing a handover procedure according to an embodiment of the disclosure will be described with reference to FIGS. 1A to 1J.

FIG. 1A shows a structure of an LTE system to which an embodiment of the disclosure may be applied.

Referring to FIG. 1A, a radio access network of the LTE system may be configured with next-generation Evolved Nodes B (ENBs) (hereinafter, also referred to as Nodes B or base stations) 1 a-05, 1 a-10, 1 a-15, and 1 a-20, a Mobility Management Entity (MME) 1 a-25, and a Serving-Gateway (S-GW) 1 a-30. A UE (hereinafter, also referred to as a terminal) 1 a-35 may be connected to an external network through the ENBs 1 a-05 to 1 a-20 and the S-GW 1 a-30.

In FIG. 1A, the ENBs 1 a-05 to 1 a-20 may correspond to existing nodes B of a Universal Mobile Telecommunication System (UMTS). The ENBs 1 a-05 to 1 a-20 may be connected to the UE 1 a-35 through a wireless channel, and perform more complicated functions than the existing nodes B. In the LTE system, because all user traffics including a real-time service such as Voice over IP (VoIP) are serviced through a shared channel, a device for performing scheduling by collecting status information of UEs, such as buffer statuses of UEs, available transmission power states of UEs, channel states of UEs, etc., is needed. The ENBs 1 a-05 to 1 a-20 may function as such a device. An ENB may generally control a plurality of cells. For example, to implement a transmission speed of 100 Mbps, the LTE system uses, as radio access technology, Orthogonal Frequency Division Multiplexing (OFDM), for example, in a bandwidth of 20 MHz. Also, Adaptive Modulation & Coding (AMC) of determining a modulation scheme and a channel coding rate according to a channel state of a UE may be applied. The S-GW 1 a-30, which is a device for providing data bearers, may generate or remove data bearers according to a control of the MME 1 a-25. The MME 1 a-25 may be in charge of various control functions, as well as a mobility management function for UEs, and may be connected to a plurality of ENBs.

FIG. 1B shows a radio protocol architecture of an LTE system to which an embodiment of the disclosure may be applied.

Referring to FIG. 1B, radio protocols of the LTE system may include Packet Data Convergence Protocols (PDCPs) 1 b-05 and 1 b-40, Radio Link Control (RLC) 1 b-10 and 1 b-35, and Medium Access Control (MAC) 1 b-15 and 1 b-30 of an UE and an ENB. The PDCPs 1 b-05 and 1 b-40 may be in charge of operations of IP header compression/restoration. Main functions of the PDCPs 1 b-05 and 1 b-40 may be summarized as follows.

-   -   Header compression and decompression: ROHC only     -   Transfer of user data     -   In-sequence delivery of upper layer PDUs at PDCP         re-establishment procedure for RLC AM     -   For split bearers in DC (only support for RLC AM): PDCP PDU         routing for transmission and PDCP PDU reordering for reception     -   Duplicate detection of lower layer SDUs at PDCP re-establishment         procedure for RLC AM     -   Retransmission of PDCP SDUs at handover and, for split bearers         in DC, of PDCP PDUs at PDCP data-recovery procedure, for RLC AM     -   Ciphering and deciphering     -   Timer-based SDU discard in uplink

The RLC 1 b-10 and 1 b-35 may reconstruct a PDCP Packet Data Unit (PDU) to an appropriate size to perform an ARQ operation, etc. Main functions of the RLC 1 b-10 and 1 b-35 may be summarized as follows.

-   -   Transfer of upper layer PDUs     -   Error Correction through ARQ (only for AM data transfer)     -   Concatenation, segmentation and reassembly of RLC SDUs (only for         UM and AM data transfer)     -   Re-segmentation of RLC data PDUs (only for AM data transfer)     -   Reordering of RLC data PDUs (only for UM and AM data transfer)     -   Duplicate detection (only for UM and AM data transfer)     -   Protocol error detection (only for AM data transfer)     -   RLC SDU discard (only for UM and AM data transfer)

RLC re-establishment

The MAC 1 b-15 and 1 b-30 may be connected to several RLC layers configured in the UE, and perform an operation of multiplexing RLC PDUs into a MAC PDU and demultiplexing RLC PDUs from a MAC PDU. Main functions of the MAC 1 b-15 and 1 b-30 may be summarized as follows.

-   -   Mapping between logical channels and transport channels     -   Multiplexing/demultiplexing of MAC SDUs belonging to one or         different logical channels into/from transport blocks (TB)         delivered to/from the physical layer on transport channels     -   Scheduling information reporting     -   Error correction through HARQ     -   Priority handling between logical channels of one UE     -   Priority handling between UEs by means of dynamic scheduling     -   MBMS service identification     -   Transport format selection     -   Padding

Physical layers (PHY) 1 b-20 and 1 b-25 may perform an operation of performing channel coding and modulation on upper layer data to generate an OFDM symbol and transmitting the OFDM symbol through a wireless channel, or an operation of demodulating an OFDM symbol received through the wireless channel, performing channel decoding on the OFDM symbol, and transferring the channel-decoded OFDM symbol to an upper layer.

FIG. 1C shows a structure of a next-generation mobile communication system to which an embodiment of the disclosure may be applied.

Referring to FIG. 1C, a radio access network of a next-generation mobile communication system (hereinafter, referred to as NR or 5G) may be configured with a next-generation New Radio Node B (NR gNB) (also, referred to as a NR base station) 1 c-10 and a New Radio Core Network (NR CR) 1 c-05. A New Radio User Equipment (NR UE) (also, referred to as a UE) 1 c-15 may be connected to an external network through the NR gNB 1 c-10 and the NR CN 1 c-05.

In FIG. 1C, the NR gNB 1 c-10 may correspond to an Evolved Node B (eNB) of an existing LTE system. The NR gNB 1 c-10 may be connected to the NR UE 1 c-15 through a wireless channel, and provide a more excellent service than an existing node B. In a next-generation mobile communication system, because all user traffics are serviced through a shared channel, a device for performing scheduling by collecting status information of UEs, such as buffer statuses of UEs, available transmission power states of UEs, channel states of UEs, etc., is needed. The NR gNB 1 c-10 may function as such a device. A NR gNB may generally control a plurality of cells. To implement ultra high-speed data transmission compared to existing LTE, OFDM as radio access technology may be combined with beam-forming technology, while providing an existing maximum bandwidth or more.

Also, AMC of determining a modulation scheme and a channel coding rate according to a channel state of a UE may be applied. The NR CN 1 c-05 may perform functions, such as mobility support, bearer setup, Quality of Service (QoS) setup, etc. The NR CN 1 c-05 may be in charge of various control functions, as well as a mobility management function for UEs, and may be connected to a plurality of base stations. Also, the next-generation mobile communication system may interwork with an existing LTE system, and the NR CN 1 c-05 may be connected to the MME 1 c-25 through a network interface. The MME 1 c-25 may be connected to an eNB 1 c-30 which is an existing base station.

FIG. 1D shows a radio protocol architecture of a next-generation mobile communication system to which an embodiment of the disclosure may be applied.

Referring to FIG. 1D, radio protocols of the next-generation mobile communication system may include NR SDAPs 1 d-01 and 1 d-45, NR PDCPs 1 d-05 and 1 d-40, NR RLC 1 d-10 and 1 d-35, and NR MAC 1 d-15 and 1 d-30 of an UE and a NR gNB.

Main functions of the NR SDAPs 1 d-01 and 1 d-45 may include some of the following functions.

-   -   Transfer of user plane data     -   Mapping between a QoS flow and a DRB for both DL and UL     -   Marking QoS flow ID in both DL and UL packets     -   Reflective QoS flow to DRB mapping for the UL SDAP PDUs

In regard of a SDAP layer as described above, the UE may receive, as an RRC message, a setting about whether to use a header of the SDAP layer or whether to use a function of the SDAP layer for each PDCP layer, for each bearer, or for each logical channel. When a SDAP header is set, the SDAP header capable of indicating the UE to update or reset mapping information about a QoS flow and a data bearer of a uplink and a downlink with a 1-bit indicator NAS reflective QoS for a NAS reflective QoS setting and a 1-bit indicator AS reflective QoS for an AS reflective QoS setting of the SDAP header may include QoS flow ID information representing QoS. QoS information described above may be used as data processing priority, scheduling information, etc. for supporting a stable service.

Main functions of the NR PDCPs 1 d-05 and 1 d-40 may include some of the following functions.

-   -   Header compression and decompression: ROHC only     -   Transfer of user data     -   In-sequence delivery of upper layer PDUs     -   Out-of-sequence delivery of upper layer PDUs     -   PDCP PDU reordering for reception     -   Duplicate detection of lower layer SDUs     -   Retransmission of PDCP SDUs     -   Ciphering and deciphering     -   Timer-based SDU discard in uplink

The reordering function of a NR PDCP device among the above-mentioned functions means a function of reordering PDCP PDUs received from a lower layer in order based on PDCP Sequence Numbers (SNs), and may include a function of transferring data to an upper layer in the reordered order. Alternatively, the reordering function of the NR PDCP device may include a function of directly transferring data regardless of order, a function of reordering and recording lost PDCP PDUs, a function of sending a status report for lost PDCP PDUs to a transmitter, and a function of requesting retransmission of lost PDCP PDUs.

Main functions of the NR RLC 1 d-10 and 1 d-35 may include some of the following functions.

-   -   Transfer of upper layer PDUs     -   In-sequence delivery of upper layer PDUs     -   Out-of-sequence delivery of upper layer PDUs     -   Error Correction through ARQ     -   Concatenation, segmentation and reassembly of RLC SDUs     -   Re-segmentation of RLC data PDUs     -   Reordering of RLC data PDUs     -   Duplicate detection     -   Protocol error detection     -   RLC SDU discard     -   RLC re-establishment

The in-sequence delivery function of a NR RLC device among the above-mentioned functions means a function of transferring RLC SDUs received from a lower layer in order to an upper layer, and may include a function of reassembling, when a RLC SDU is segmented into several RLC SDUs and received, the RLC SDUs and transferring a result of the reassembling, a function of reordering received RLC PDUs based on RLC SNs or PDCP SNs, a function of reordering and recording lost RLC PDUs, a function of sending a status report for lost RLC PDUs to a transmitter, a function of requesting retransmission of lost RLC PDUs, a function of transferring, when there are lost RLC SDUs, RLC SDUs before the lost RLC SDUs in order to the upper layer, a function of transferring, when a preset timer has been expired although there are lost RLC SDUs, all RLC SDUs received before the preset timer starts in order to the upper layer, or a function of transferring, when the preset time has been expired although there are lost RLC SDUs, all RLC SDUs received up to now in order to the upper layer.

Also, RLC PDUs may be processed in order (in arrival order, regardless of SNs or order of SNs) in which they are received, and transferred to the PDCP device out of sequence delivery, and in the case of segments, segments which have been stored in a buffer or which are to be received later may be received and reconstructed as a compete RLC PDU and then processed and transferred to the PDCP device. A NR RLC layer as described above may include no concatenation function, and the above-described function may be performed in a NR MAC layer or replaced with a multiplexing function of the NR MAC layer.

The out-of-sequence delivery function of the NR RLC device as described above means a function of transferring RLC SDUs received from a lower layer to the upper layer regardless of order, and may include a function of reassembling, when a RLC SDU is segmented into several RLC SDUs and received, the RLC SDUs and transferring a result of the reassembling, and a function of storing and ordering RLC SNs or PDCP SNs of the received RLC PDUs and recording lost RLC PDUs.

The NR MAC 1 d-15 and 1 d-30 may be connected to several NR RLC layers configured in the UE, and main functions of the NR MAC 1 d-15 and 1 d-30 may include some of the following functions.

-   -   Mapping between logical channels and transport channels     -   Multiplexing/demultiplexing of MAC SDUs     -   Scheduling information reporting     -   Error correction through HARQ     -   Priority handling between logical channels of one UE     -   Priority handling between UEs by means of dynamic scheduling     -   MBMS service identification     -   Transport format selection     -   Padding

NR PHY layers 1 d-20 and 1 d-25 may perform an operation of performing channel coding and modulation on upper layer data to generate an OFDM symbol and transferring the OFDM symbol through a wireless channel, or performing demodulation and channel decoding on an OFDM symbol received through the wireless channel and transferring a result of the modulation and channel decoding to an upper layer.

FIG. 1E is a view for describing a procedure in which a gNB according to an embodiment of the disclosure releases a connection of an UE for the UE to change from an RRC connected mode to an RRC idle mode and a procedure in which the UE sets up a connection to the gNB to change from the RRC idle mode to the RRC connected mode.

Referring to FIG. 1E, when the UE transmitting and receiving data receives or transmits no data for a certain reason or for a preset time in an RRC connected mode, the gNB may transmit an RRCConnectionRelease message to the UE to change the UE to the RRC idle mode, in operation 1 e-01. The UE (hereinafter, also referred to as an idle mode UE) in which no connection is currently established may perform, when data to be transmitted is generated, an RRC connection establishment process together with the gNB. The UE may establish reverse transmission synchronization with the gNB through a random access process, and transmit an RRCConnectionRequest message to the gNB, in operation 1 e-05. The RRCConnectionRequest message may include an identifier of the UE and a cause establishmentCause for connection establishment.

The gNB may transmit an RRCConnectionSetup message to the UE to enable the UE to establish an RRC connection, in operation 1 e-10. The RRCConnectionSetup message may include RRC connection configuration information, etc. Also, the RRCConnectionSetup message may include at least one information of bearer configuration information of the UE, configuration information of a PHY layer, configuration information of a MAC layer, configuration information of a RLC layer, configuration information of a PDCP layer, or configuration information of a SDAP layer.

Also, the configuration information of the PDCP layer may include an indicator indicating whether to trigger a PDCP status report in an RLC UM (unacknowledged mode) mode or an RLC AM (acknowledged mode) mode. The indicator may be an indicator indicating the PDCP layer to generate, configure, and transmit a PDCP status report when a PDCP re-establishment procedure is performed. When the above-described indicator is included in an RRC message, the UE may trigger, configure, and transmit a PDCP status report for a bearer corresponding to the PDCP layer. According to another method, when the above-described indicator is set, the UE may trigger, configure, and transmit a PDCP status report at the PDCP layer in the case that a conditional handover is indicated. According to another method, when the above-described indicator is set, the UE may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report to a target gNB in the case that a handover is indicated. According to another method, when the above-described indicator is set, the UE may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report to a source gNB in the case that a handover is indicated.

An RRC connection is also referred to as a Signaling Radio Bearer (SRB), and may be used to transmit and receive an RRC message which is a control message between a UE and a gNB. The UE which has established an RRC connection may transmit an RRCConnetionSetupComplete message to the gNB, in operation 1 e-15. The RRCConnetionSetupComplete message may include a SERVICE REQUEST message as a control message through which the UE requests the MME to set a bearer for a preset service.

The gNB may transmit the SERVICE REQUEST message included in the RRCConnetionSetupComplete message to an Access and Mobility Management Function (AMF), in operation 1 e-20, and the AMF may determine whether to provide the preset service requested by the UE. When the AMF determines to provide the preset service requested by the UE, the AMF may transmit an INITIAL CONTEXT SETUP REQUEST message to the gNB, in operation 1 e-25. The INITIAL CONTEXT SETUP REQUEST message may include QoS information to be applied for a Data Radio Bearer (DRB) setting, security-related information (for example, a security key or a security algorithm) to be applied to the DRB, etc.

The gNB may exchange a SecurityModeCommand message with a SecurityModeComplete message to set security with the UE, in operations 1 e-30 and 1 e-35. When a security setting is completed, the gNB may transmit an RRCConnectionReconfiguration message to the UE, in operation 1 e-40.

Also, the RRCConnectionReconfiguration message may include bearer configuration information, configuration information of the PHY layer, configuration information of the MAC layer, configuration information of the RLC layer, configuration information of the PDCP layer, or configuration information of the SDAP layer.

Also, the configuration information of the PDCP layer may include an indicator indicating whether to trigger a PDCP status report in the RLC UM mode or the RLC AM mode. The indicator may indicate the PDCP layer to generate, configure, and transmit a PDCP status report when a PDCP re-establishment procedure is performed. When the indicator is included in an RRC message, the UE may trigger, configure, and transmit a PDCP status report for a bearer corresponding to the PDCP layer. According to another method, when the indicator is set, the UE may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report, in the case that a conditional handover is indicated. According to another method, when the indicator is set, the UE may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report to a target gNB, in the case that a handover is indicated. According to another method, when the indicator is set, the UE may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report to a source gNB, in the case that a conditional handover is indicated.

The RRCConnectionReconfiguration message may include configuration information of a DRB by which user data is to be processed, and the UE may apply the configuration information of the DRB to set up the DRB, and transmit an RRCConnectionReconfigurationComplete message to the gNB, in operation 1 e-45.

The gNB may set up the DRB with the UE and then transmit an initial context setup complete message to the AMF, in operation 1 e-50. The AMF may receive the initial context setup complete message, and perform a session management procedure with a User Plane Function (UPF) to establish a PDU session, in operation 1 e-60. When the above-described process is completed, the UE may transmit/receive data to/from the gNB through the UPF, in operations 1 e-65 and 1 e-70.

As described above, a general data transmission process may be configured with three operations of RCC connection setup, security setup, and DRB setup. Also, the gNB may transmit an RRCConnectionReconfiguration message to newly set up, add, or change a setting to the UE for a certain reason, in operation 1 e-75.

According to an embodiment of the disclosure, the bearer may have a meaning including a SRB and a DRB, wherein the SRB means a Signaling Radio Bearer and the DRB means a Data Radio Bearer. The SRB may be mainly used to transmit and receive RRC messages of the RRC layer, and the DRB may be mainly used to transmit and receive user layer data. Also, UM DRB means a DRB using the RLC layer operating in an Unacknowledged Mode (UM), and AM DRB means a DRB using the RLC layer operating in an Acknowledged Mode (AM).

FIG. 1F shows a handover procedure in a next-generation mobile communication system according to an embodiment of the disclosure.

Referring to FIG. 1F, a UE 1 f-01 which is in the RRC connected mode may send a cell measurement report to a source gNB 1 f-02 periodically or when a preset event is satisfied, in operation 1 f-05. The source gNB 1 f-02 may determine whether to perform a handover of the UE 1 f-01 to an adjacent cell based on the cell measurement information. The handover may be a technique of changing a source gNB providing a service to a UE being in a connected mode to another gNB (or another cell in the same gNB).

When the source gNB 1 f-02 determines a handover, the source gNB 1 f-02 may send a handover (HO) request message to a new gNB, that is, a target gNB 1 f-03 that is to provide a service to the UE 1 f-01, thereby requesting a handover, in operation 1 f-10. When the target gNB 1 f-03 accepts the handover request, the target gNB 1 f-03 may transmit a HO request Ack message to the source gNB 1 f-02, in operation 1 f-15. When the source gNB 1 f-02 receives the HO request Ack message, the source gNB 1 f-02 may transmit a HO command message to the UE 1 f-01, in operation 1 f-20. The source gNB 1 f-02 may transfer the HO command message to the UE 1 f-01 by using an RRC Connection Reconfiguration message, in operation 1 f-20. When the UE 1 f-01 receives the HO command message, the UE 1 f-01 may stop data transmission/reception to/from the source gNB 1 f-02, and start a T304 timer, in operation 1 f-25. The T304 timer may return the UE 1 f-01 to its original setting and change the UE 1 f-01 to an RRC idle state, when the UE 1 f-01 fails to perform a handover to the target gNB 1 f-03 within a preset time period. The source gNB 1 f-02 may transfer a sequence number (SN) status about a transmission/reception status of uplink or downlink data to the target gNB 1 f-03, and when there is downlink data, the source gNB 1 f-02 may transfer the downlink data to the target gNB 1 f-03, in operations 1 f-30 and 1 f-35. The UE 1 f-01 may attempt a random access to a target cell indicated from the source gNB 1 f-02, in operation 1 f-40. The random access may be aimed to notify the target cell that the UE 1 f-01 moves through a handover, while synchronizing an uplink. For the random access, the UE 1 f-01 may transmit a preamble corresponding to a preamble ID received from the source gNB 1 f-02 or a randomly selected preamble ID to the target cell. When a preset number of subframes elapse after the preamble is transmitted, the UE 1 f-01 may monitor whether a Random Access Response (RAR) message is transmitted from the target cell. A time period in which the UE 1 f-01 monitors whether the RAR message is transmitted is referred to as a RAR window. When a RAR message is received within the time period of the monitoring in operation 1 f-45, the UE 1 f-01 may transmit a HO complete message as an RRC Reconfiguration Complete message to the target gNB 1 f-03, in operation 1 f-55. When a random access response is successfully received from the target gNB 1 f-03, the UE 1 f-01 may stop the T304 timer, in operation 1 f-50.

To correct paths of bearers set to the source gNB 1 f-02, the target gNB 1 f-03 may request a core network 1 f-04 to correct paths in operations 1 f-60 and 1 f-65, and notify the source gNB 1 f-02 of discarding UE context of the UE 1 f-01, in operation 1 f-70. Accordingly, the UE 1 f-01 may attempt to receive data with respect to the target gNB 1 f-03 from when the RAR window starts, and after the RAR message is received, the UE 1 f-01 may transmit the RRCConnectionReconfigurationComplete message to the target gNB 1 f-03 and start data transmission/reception to/from the target gNB 1 f-30. Then, when the source gNB 1 f-02 receives an end-marker indicating final data from the core network 1 f-04, the source gNB 1 f-02 may determine that a path of downlink data for the UE 1 f-01 has changed to the target gNB 1 f-03, and perform forwarding of the final data to the target gNB 1 f-30. According to an embodiment of the disclosure, the core network 1 f-04 may include at least one of a MME, a S-GW, or an ANTE, and further include at least one of a UPF or a SMF.

In the above-described handover procedure, when the source gNB 1 f-02 transmits the HO command message (for example, an RRC Reconfiguration message) to the UE 1 f-01, at least one of information about a target gNB to which the UE 1 f-01 needs to perform a handover, information about a random access transmission resource for a cell, preamble information, bearer configuration information, or cell information may be set in the HO command message.

Also, the HO command message may include an indicator defined to indicate what kind of handover the UE 1 f-01 needs to perform. For example, the source gNB 1 f-02 may define indicators for a plurality of handover methods, and set one of the defined indicators such that the UE 1 f-01 perform a handover method corresponding to the set indicator. The plurality of handover methods may include at least one method of a RACH-less handover method, a make before break (MBB) handover method, an enhanced make before break (eMBB) method, or a conditional handover method. Also, the HO command message may include an indicator indicating whether or not the UE 1 f-01 generates a PDCP status report for each bearer (UM DRB or AM DRB) or the PDCP layer and transmits the PDCP status report to the source gNB 1 f-02 or the target gNB 1 f-03.

FIG. 1G shows a conditional handover procedure according to an embodiment of the disclosure.

In FIG. 1G, a UE 1 g-01 which is in the RRC connected mode may send a cell measurement report to a source gNB 1 g-02 periodically or when a preset event is satisfied, in operation 1 g-05. The source gNB 1 g-02 may determine whether to perform a conditional handover of the UE 1 g-01 to adjacent cells, based on the cell measurement information.

The conditional handover may be a technique of changing a source gNB providing a service to a UE being in a connected mode to another gNB (or another cell in the same gNB). For conditional handover a plurality of target gNB candidates and conditions for determining a target gNB may be set for a UE to enable the UE to select, as a target gNB, a target gNB candidate from among the plurality of target gNB candidates according to the set conditions and perform a handover. As the conditions for determining the target gNB, various conditions, such as a greater frequency signal intensity than a threshold value, a first synchronized signal frequency, a determination according to a UE's implementation without any condition, etc. may be set.

When the source gNB 1 g-02 determines performing of a conditional handover, the source gNB 1 g-02 may send HO request messages to a plurality of target gNB candidates including a plurality of new gNBs that are to provide a service to the UE 1 g-01, that is, a plurality of target gNBs Target gNB 1 1 g-03 and Target gNB 2 1 g-04, thereby requesting a handover, in operation 1 g-10. When the plurality of target gNBs 1 g-03 and 1 g-04 accept the handover request, the plurality of target gNBs 1 g-03 and 1 g-04 may transmit HO request Ack messages to the source gNB 1 g-02, in operation 1 g-15. When the source gNB 1 g-02 receives the HO request Ack messages, the source gNB 1 g-02 may transmit a HO command message to the UE 1 g-01, in operation 1 g-20. The source gNB 1 g-02 may transfer the HO command message to the UE 1 g-01 by using an RRC Connection Reconfiguration message, in operation 1 g-20.

In the above-described handover procedure, when the source gNB 1 g-02 transmits the HO command message (for example, an RRC Reconfiguration message) to the UE 1 g-01, at least one of information about the plurality of target gNBs 1 g-03 and 1 g-04 to which the UE 1 g-01 needs to perform a handover, information about a random access transmission resource for a cell, preamble information, bearer configuration information, or cell information may be set in the HO command message.

Also, the HO command message may include an indicator defined to indicate what kind of handover the UE 1 g-01 needs to perform. For example, the source gNB 1 g-02 may define indicators for a plurality of handover methods, and set one of the defined indicators such that the UE 1 g-01 performs a handover method corresponding to the set indicator. The plurality of handover methods may include at least one method of a RACH-less handover method, a MBB handover method, an eMBB method, or a conditional handover method. Also, the HO command message may include an indicator indicating whether or not the UE 1 g-01 generates a PDCP status report for each bearer (UM DRB or AM DRB) or the PDCP layer and transmits the PDCP status report to the source gNB 1 g-02 or the target gNB.

When the source gNB 1 g-02 indicates a conditional handover, the HO command message may include a condition for enabling the UE 1 g-01 to select a target gNB from among the plurality of target gNBs 1 g-03 and 1 g-04. As the condition for enabling the UE 1 g-01 to select the target gNB, various conditions, such as a greater frequency signal intensity than a threshold value, a first synchronized signal frequency, a determination according to implementation of the UE 1 g-01 without any condition, etc. may be set.

When the UE 1 g-01 receives the HO command message, the UE 1 g-01 may start a T304 timer and perform a conditional handover procedure. More specifically, the UE 1 g-01 may start a cell re-selection procedure or a cell search procedure for the plurality of target gNBs 1 g-03 and 1 g-04 or a plurality of frequencies according to the condition included in the HO command message or the implementation of the UE 1 g-01, in operation 1 g-25.

Then, when the UE 1 g-01 determines a target gNB (for example, the target gNB 1 g-03) or a cell that is suitable for the condition or the implementation of the UE 1 g-01, the UE 1 g-01 may attempt a random access to the determined target gNB 1 g-03 or the determined cell (also, referred to as a target cell, in operation 1 g-40. The random access may be aimed to inform the target cell that the UE 1 g-01 moves through a handover, while synchronizing an uplink. For the random access, the UE 1 g-01 may transmit a preamble corresponding to a preamble ID received from the source gNB 1 g-02 or a randomly selected preamble ID to the target cell. When a preset number of subframes elapse after the preamble is transmitted, the UE 1 g-01 may monitor whether a RAR message is transmitted from the target cell. A time period in which the UE 1 g-01 monitors whether the RAR message is transmitted is referred to as a RAR window. When the RAR message is received within the time period of the monitoring in operation 1 g-45, the UE 1 g-01 may transmit a HO complete message as an RRCConnectionReconfigurationComplete message to the target gNB 1 g-03, in operation 1 g-55. When a random access response is successfully received from the target gNB 1 g-03, the UE 1 g-01 may stop the T304 timer, in operation 1 g-50.

To correct paths of bearers set to the source gNB 1 g-02, the target gNB 1 g-03 may request route correction, and notify the source gNB 1 g-02 of discarding UE context of the UE 1 g-01. Accordingly, the UE 1 g-01 may attempt to receive data with respect to the target gNB 1 g-03 from when the RAR window starts, and after the RAR message is received, the UE 1 g-01 may transmit the RRCConnectionReconfigurationComplete message to the target gNB 1 g-03 and start data transmission/reception to/from the target gNB 1 g-30.

In the conditional handover method according to an embodiment of the disclosure as described above with reference to FIG. 1G, a time at which the UE 1 g-01 releases a connection to the source gNB 1 g-02 may be determined by the UE 1 g-01 based on a first condition described below. The first condition is that when at least one condition among a plurality of conditions described below is satisfied, the UE 1 g-01 may release a connection to the source gNB 1 g-02.

-   -   When a UE performs a procedure of a random access to a target         gNB and receives a random access response from the target gNB     -   When a UE performs a procedure of a random access to a target         gNB, receives a random access response from the target gNB,         configures a handover complete message, and transmits the         handover complete message to the target gNB     -   When a UE completes a procedure of a random access to a target         gNB and is first assigned a PUCCH or PUSCH uplink transmission         resource or when a UE first transmits data in a PUCCH or PUSCH         uplink transmission resource     -   When a gNB sets a separate timer through an RRC message for a UE         and the timer is expired     -   The timer may start at at least one time of a time at which the         UE receives a handover command message from a source gNB, a time         at which the UE starts a random access to the target gNB (a time         at which the UE transmits a preamble to the target gNB), a time         at which the UE receives a random access response from the         target gNB, a time at which the UE transmits a handover complete         message to the target gNB, or a time at which the UE first         transmits data in a PUCCH or PUSCH uplink transmission resource.     -   When a UE performs a procedure of a random access to a target         gNB, receives a random access response from the target gNB,         configures a handover complete message, and transmits the         handover complete message to the target gNB, and then a         successful transfer of the handover complete message is         acknowledged by a MAC layer (HARQ ACK) or a RLC layer (RLC ACK)     -   When a UE performs a procedure of a random access to a target         gNB, receives a random access response from the target gNB,         configures a handover complete message, transmits the handover         complete message to the target gNB, and then is first assigned         or indicated a uplink transmission resource from the target gNB.

The conditional handover method according to an embodiment of the disclosure as described above with reference to FIG. 1G may have two issues.

A first issue of the issues is that the source gNB 1 g-02 is not capable of accurately recognizing a time at which the UE 1 g-01 releases a connection to the source gNB 1 g-02, in the conditional handover method. The reason is because, in a conditional handover, the UE 1 g-01 selects a gNB from among a plurality of target gNBs according to a preset condition and itself releases a connection to the source gNB 1 g-02 according to the first condition described above. Accordingly, the source gNB 1 g-02 may transmit downlink data to the UE 1 g-01 even after the UE 1 g-01 releases a connection to the source gNB 1 g-02. Therefore, massive downlink data loss may occur.

A second issue of the issues is that, in a conditional handover, the source gNB 1 g-02 transmits a handover command message including conditions and configuration information about a plurality of target gNBs to the UE 1 g-01 so that the source gNB 1 g-02 is not capable of expecting a target gNB to which the UE 1 g-01 will perform a handover.

In the handover procedure described above with reference to FIG. 1F, because the source gNB 1 f-02 transmits a handover command message including information about the single target gNB 1 f-03 to the UE 1 f-01, the source gNB 1 f-02 may expect that the UE 1 f-01 will perform a handover to the target gNB 1 f-03. Accordingly, referring to FIG. 1F, the source gNB 1 g-02 may transmit a handover command message to the UE 1 g-01, and then immediately start transmitting uplink or downlink data to the target gNB 1 f-03 together with a transmission/reception status or a SN status of the uplink or downlink data.

However, in the conditional handover shown in FIG. 1G, although the UE 1 g-01 completes a handover to the target gNB 1 g-03 among the plurality of target gNBs 1 g-03 and 1 g-04, the source gNB 1 g-02 may be not capable of recognizing the target gNB 1 g-03 to which the UE 1 g-01 performs a handover until the target gNB 1 g-03 indicates the source gNB 1 g-02 of a successful handover of the UE 1 g-01. Accordingly, the source gNB 1 g-02 may start late performing a procedure of transmitting uplink or downlink data to the target gNB 1 f-03 together with a transmission/reception status or a SN status of the uplink or downlink data, resulting in a transmission delay.

Hereinafter, a first embodiment of a conditional handover operation for overcoming the first issue that may be occurred in a conditional handover is proposed in detail.

In a handover procedure, when the UE 1 g-01 receives a handover command message and the handover command message indicates a conditional handover procedure, the following operations may be performed.

-   -   The UE 1 g-01 may complete a procedure for a handover to a         target gNB with respect to a UM bearer (or UM DRB) or an AM         bearer (or AM DRB) to prevent downlink data from being lost in a         conditional handover procedure, trigger, configure, and generate         a PDCP status report for the downlink data at a PDCP layer of         the UM bearer or the AM bearer, and transmit the PDCP status         report to the target gNB when transmitting uplink data (for         example, a handover complete message 1 g-55 or uplink data) to         the target gNB.

When a handover command message is received, the PDCP layer may trigger a PDCP re-establishment procedure, discard previously generated data (for example, old PDCP PDU), newly apply header compression, integrity protection, or a ciphering procedure, with a new security key, to data (for example, PDCP SDU) not yet transmitted among uplink data to generate data (for example, PDCP PDU), and transmit the generated data to the target gNB after a handover to the target gNB is completed.

When the handover command message includes a PDCP status report indicator, the UE 1 g-01 may trigger and configure a PDCP status report for the corresponding bearer at the PDCP layer and transmit the PDCP status report. According to another method, when a PDCP status report indicator is set, the UE 1 g-01 may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report, in the case that a conditional handover is indicated. According to another method, when a PDCP status report indicator is set, the UE 1 g-01 may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report to the target gNB, in the case that a handover is indicated. According to another method, when a PDCP status report indicator is set, the UE 1 g-01 may trigger and configure a PDCP status report at the PDCP layer and transmit the PDCP status report to the source gNB 1 g-02, in the case that a handover is indicated. A time at which the PDCP layer triggers a PDCP status report when a PDCP status report indicator is set may be one of times included in the first condition described above, when a connection to the source gNB 1 g-02 is released, or when a handover to the target gNB is successfully performed.

-   -   When the target gNB receives a PDCP status report for downlink         data with respect to the UM bearer or the AM bearer from the UE         1 g-01, the target gNB may retransmit data for which a PDCP         discard timer has not yet been expired with respect to the UM         bearer (or in consideration of a transmission time) or data of         which successive reception has not been confirmed, to the UE 1         g-01 through a downlink, based on the PDCP status report,         thereby reducing data loss.

Also, for example, the target gNB may discard data (for example, PDCP PDU or PDCP SDU) of which successful transfer has been confirmed from the PDCP status report with respect to the AM bearer, and selectively retransmit data (for example, PDCP PDU or PDCP SDU) of which successful transfer has not been confirmed, thereby reducing data loss.

-   -   Also, the UE 1 g-01 may not perform data retransmission until         receiving a PDCP status report for uplink data from the target         gNB after completing a conditional handover, to prevent         unnecessary waste of transmission resources, that is, to prevent         waste of transmission resources that are caused by accumulative         retransmission in a PDCP re-establishment procedure of the UE 1         g-01. Although the UE 1 g-01 does not perform data         retransmission until receiving a PDCP status report, the UE 1         g-01 may newly apply header compression, integrity protection,         or a ciphering procedure to data not yet transmitted, with a new         security key, to generate data (for example, PDCP PDU) and         transmit the data to the target gNB. Also, after the UE 1 g-01         receives a PDCP status report for uplink data from the target         gNB, the UE 1 g-01 may newly apply header compression, integrity         protection, or a ciphering procedure to data (for example, PDCP         SDU) of which successful transfer has not been confirmed from         the PDCP status report, with a new security key, to generate         data (for example, PDCP PDU), and transmit the data to the         target gNB after a handover to the target gNB is completed.         According to another method, the target gNB may not assign         uplink transmission resource to the UE 1 g-01 until a PDCP         status report for uplink data is configured and transmitted.

Hereinafter, a second embodiment of a conditional handover operation for overcoming the first issue that may be generated in a conditional handover is proposed in detail.

The first issue of the conditional handover method proposed in an embodiment of the disclosure is that the source gNB 1 g-02 is not capable of accurately recognizing a time at which the UE 1 g-01 releases a connection to the source gNB 1 g-02. The reason is because, in a conditional handover, the UE 1 g-01 selects a target gNB from among the plurality of target gNBs 1 g-03 and 1 g-04 according to a preset condition and itself releases a connection to the source gNB 1 g-02 according to the first condition described above. Accordingly, according to the second embodiment of the disclosure, when the UE 1 g-01 releases a connection to the source gNB 1 g-02 according to the first condition described above, the UE 1 g-01 may indicate the source gNB 1 g-02 of a connection release. Detailed operations of the second embodiment may be as follows.

When the UE 1 g-01 receives a handover command message in a handover procedure and the handover command message indicates a conditional handover procedure, the following operations may be performed.

-   -   When the UE 1 g-01 releases a connection to the source gNB 1         g-02 according to the first condition, the UE 1 g-01 may         transmit an indication indicating that a connection to the         source gNB 1 g-02 has been released to the source gNB 1 g-02.     -   The indication indicating that the connection to the source gNB         1 g-02 has been released may be transmitted when the UE 1 g-01         transmits a new RRC message to the source gNB 1 g-02 or defines         a new indicator in an existing RRC message and transmits the         resultant RRC message to the source gNB 1 g-02 or when the UE 1         g-01 defines a new MAC CE, a new RLC control PDU, or a new PDCP         control PDU and transmits the new MAC CE, the new RLC control         PDU, or the new PDCP control PDU to the source gNB 1 g-02.

In the case of the new MAC CE, when the UE 1 g-01 releases a connection to the source gNB 1 g-02, the UE 1 g-01 may configure the new MAC CE and transmit the new MAC CE to the source gNB 1 g-02 to indicate a connection release from the source gNB 1 g-02 and confirm whether HARQ ACK/NACK of MAC PDU included in the MAC CE is successfully transferred. When the UE 1 g-01 defines a new RLC control PDU or a new PDCP control PDU and indicates a connection release from the source gNB 1 g-02 by using the new RLC control PDU or the new PDCP control PDU, the UE 1 g-1 may transmit a RLC control PDU or a PDCP control PDU newly defined for each bearer. The newly defined RRC message, the newly defined indicator of the RRC message, the newly defined MAC CE, the newly defined RLC control PDU, or the newly defined PDCP control PDU may include an indicator indicating a target gNB to which the UE 1 g-01 performs, attempts, or completes a handover among the plurality of target gNB candidates in the conditional handover method. The indicator indicating the target gNB may be at least one of an identifier of the target gNB, a bitmap of the target gNB, or an indicator indicating a certain target of the plurality of target gNB candidates.

-   -   According to another method, when the UE 1 g-01 releases a         connection to the source gNB 1 g-02 according to the first         condition described above, the UE 1 g-01 may transmit a PDCP         status report to the source gNB 1 g-02 for each bearer (UM         bearer or AM bearer), wherein a 1-bit indicator indicating a         connection release may be defined and used in the PDCP status         report. When the source gNB 1 g-02 receives a PDCP status report         for each bearer from the UE 1 g-01 after indicating a         conditional handover, the source gNB 1 g-02 may consider that         the PDCP status report itself indicates a connection release of         the UE 1 g-01 from the source gNB 1 g-02 and has been finally         transmitted from the UE 1 g-01. For example, when the source gNB         1 g-02 receives a PDCP status report for all DRBs from the UE 1         g-01, receives a PDCP status report for all AM DRBs from the UE         1 g-01, or receives a PDCP status report from at least one         bearer after indicating the UE 1 g-01 to perform a conditional         handover, the source gNB 1 g-02 may consider that the UE 1 g-01         has released a connection to the source gNB 1 g-02.     -   Therefore, according to the second embodiment described above,         when the source gNB 1 g-02 confirms that the UE 1 g-01 has         released a connection to the source gNB 1 g-02, the source gNB 1         g-02 may be prevented from transmitting unnecessary downlink         data to the UE 1 g-01 to avoid data loss, may prepare         downlink/uplink data which the UE 1 g-01 is to transfer with a         target gNB to which the UE 1 g-01 has completed a handover or a         SN status of the downlink/uplink data, and may start         transmitting the SN status or the downlink/uplink data.     -   The newly defined RRC message, the newly defined indicator of         the RRC message, the newly defined MAC CE, the newly defined RLC         control PDU, or the newly defined PDCP control PDU may include         an indicator indicating a target gNB to which the UE 1 g-01         performs, attempts, or completes a handover among the plurality         of target gNB candidates in the conditional handover method when         a connection release from the source gNB 1 g-02 is indicated.         The indicator indicating the target gNB may be at least one of         an identifier of the target gNB, a bitmap of the target gNB, or         an indicator indicating a certain target of the plurality of         target gNB candidates. Accordingly, when the source gNB 1 g-02         receives an indicator indicating a target gNB from the UE 1         g-01, the source gNB 1 g-02 may immediately prepare         downlink/uplink data that is to be transferred to the target gNB         or a SN status of the downlink/uplink data, and start         transmitting the SN status or the downlink/uplink data to the         target gNB, thereby reducing a transmission delay. Therefore,         according to the second embodiment, the second issue of the         conditional handover method may be overcome.

Hereinafter, a third embodiment of a conditional handover operation for overcoming the second issue that may be generated in a conditional handover is proposed in detail.

The second issue is that, in a conditional handover, the source gNB 1 g-02 transmits a handover command message including conditions and configuration information about a plurality of target gNBs to the UE 1 g-01 so that the source gNB 1 g-02 is not capable of expecting a target gNB to which the UE 1 g-01 will perform a handover. However, in the conditional handover, although the UE 1 g-01 completes a handover to a target gNB among the plurality of target gNB candidates, the source gNB 1 g-02 may be not capable of recognizing the target gNB to which the UE 1 g-01 performs a handover until the target gNB indicates the source gNB 1 g-02 of a successful handover of the UE 1 g-01. Accordingly, the source gNB 1 g-02 may start late a procedure of transferring uplink or downlink data to the target gNB together with a transmission/reception status or a SN status of the uplink or downlink data, resulting in a transmission delay.

The third embodiment of the conditional handover operation for overcoming the second issue that may be generated in the conditional handover according to an embodiment of the disclosure is as follows.

-   -   In FIG. 1G, when the UE 1 g-01 indicates a connection release         from the source gNB 1 g-02 as described above in the second         embodiment as soon as the source gNB 1 g-02 transmits a handover         command message to the UE 1 g-01 in operation 1 g-20 or after         the source gNB 1 g-02 transmits a handover command message to         the UE 1 g-01, the source gNB 1 g-02 may start transferring         uplink or downlink data and a SN (PDCP SN) status report for the         uplink or downlink data in duplicate to the plurality of target         gNB candidates. That is, in the third embodiment, the source gNB         1 g-02 may transfer uplink or downlink data and a SN status in         duplicate to the plurality of target gNB s.     -   In the third embodiment, when a target gNB candidate of the         plurality of target gNB candidates confirms that the UE 1 g-01         has performed a handover to itself, the target gNB candidate may         check lost data and retransmit the lost data without any         transmission delay by using the SN status and uplink and         downlink data transferred from the source gNB 1 g-02 to reduce         data loss, and perform data transmission/reception to/from the         UE 1 g-01.     -   Also, the remaining target gNB candidates to which the UE 1 g-01         has never performed a handover among the plurality of target gNB         candidates may discard the SN status and uplink or downlink data         transferred from the source gNB 1 g-02. For example, after the         plurality of target gNB candidates transmit a message indicating         allowance of a handover to the source gNB 1 g-02 or when the         plurality of target gNB candidates receive a SN status and         uplink or downlink data from the source gNB 1 g-02, the target         gNB candidates may start a timer. When the target gNB candidates         determine that the UE 1 g-01 attempts no access for a handover         to the target gNB candidates until the timer is expired, the         target gNB candidates may determine that the UE 1 g-01 has         performed a procedure of a handover to another target gNB         candidate, and discard the SN status and uplink or downlink data         transferred from the source gNB 1 g-02. According to another         method, a target gNB to which the UE 1 g-01 has completed a         handover may indicate the source gNB 1 g-02 that the UE 1 g-01         has completed a handover, and the source gNB 1 g-02 may indicate         the remaining target gNB candidates to which the UE 1 g-01 has         not performed a handover to discard the SN status and uplink or         downlink data transferred from the source gNB 1 g-02.

Hereinafter, a fourth embodiment of a conditional handover operation for overcoming the second issue that may be generated in the conditional handover according to an embodiment of the disclosure is as follows.

-   -   In FIG. 1G, the source gNB 1 g-02 may indicate that the UE 1         g-01 releases a connection to the source gNB 1 g-02 as in the         second embodiment described above as soon as the source gNB 1         g-02 transmits a handover command message to the UE 1 g-01 in         operation 1 g-20 or after the source gNB 1 g-02 transmits a         handover command message to the UE 1 g-01. And the UE 1 g-01 may         indicate the source gNB 1 g-02 of a target gNB to which the UE         1G-01 performs or completes a handover as in the second         embodiment described above. Then the source gNB 1 g-02 may start         transferring uplink or downlink data and a SN (PDCP SN) status         report about the uplink or downlink data to the target gNB         indicated in the second embodiment or the target gNB to which         the UE 1 g-01 performs the handover.     -   In the fourth embodiment, the source gNB 1 g-02 may transmit the         uplink or downlink data and the SN status to the target gNB,         based on the indication about the connection release of the UE 1         g-01 from the source gNB 1 g-02, received from the UE 1 g-01 and         the indication about the target gNB to which the UE 1 g-01         performs the handover, by using the second embodiment described         above. Accordingly, the target gNB may check lost data and         retransmit the lost data without any transmission delay by using         the SN status and uplink and downlink data to reduce data loss,         and perform data transmission/reception to/from the UE 1 g-01.

The UE 1 g-01 may perform at least one of the first embodiment, the second embodiment, the third embodiment, or the fourth embodiment of the efficient conditional handover methods proposed in the disclosure, when performing a conditional handover. For example, the UE 1 g-01 may perform all of the first to fourth embodiments or some of the above-described embodiments together.

FIG. 1H shows operations of a conditional handover procedure of a UE according to an embodiment of the disclosure.

Referring to FIG. 1H, a UE 1 h-01 may receive a handover command message from a source gNB, in operation 1 h-05. The UE 1 h-01 may check an indicator indicating a conditional handover from the handover command message, and determine whether a conditional handover is indicated, in operation 1 h-10. When the handover command message indicates a conditional handover method, the UE 1 h-01 may perform a conditional handover method according to an embodiment of the disclosure, in operation 1 h-15. For example, the UE 1 h-01 may perform at least one of the first embodiment, the second embodiment, the third embodiment, or the fourth embodiment. The UE 1 h-01 may perform all of the first to fourth embodiments or some of the above-described embodiments together.

FIG. 1I shows a structure of a UE or a wireless node according to an embodiment of the disclosure.

Referring to FIG. 1I, the UE may include a Radio Frequency (RF) processor 1 i-10, a baseband processor 1 i-20, a storage device 1 i-30, and a controller 1 i-40. However, an internal structure of the UE is not limited to the embodiment shown in FIG. 1I, and the UE may include more or less components than those shown in FIG. 1I. The RF processor 1 i-10 may perform a function for transmitting and receiving signals through a wireless channel, such as band conversion, amplification, etc. of signals. That is, the RF processor 1 i-10 may up-convert a baseband signal provided from the baseband processor 1 i-20 into a RF band signal, transmit the RF band signal through an antenna, and down-convert an RF band signal received through the antenna into a baseband signal. For example, the RF processor 1 i-10 may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a digital-to-analog converter (DAC), an analog-to-digital converter (ADC), etc. In FIG. 1I, a single antenna is shown, however, the UE may include a plurality of antennas. Also, the RF processor 1 i-10 may include a plurality of RF chains. Furthermore, the RF processor 1 i-10 may perform beamforming. For beamforming, the RF processor 1 i-10 may adjust phases and magnitudes of signals that are transmitted/received through the plurality of antennas or antenna elements. Also, the RF processor 1 i-10 may perform MIMO, and when MIMO is performed, the RF processor 1 i-10 may receive several layers. The RF processor 1 i-10 may perform sweeping of a received beam by appropriately setting the plurality of antennas or antenna elements according to a control of the controller 1 i-40, or may adjust a direction and width of a received beam such that the received beam is coordinated with a beam that is to be transmitted.

The baseband processor 1 i-20 may perform a function of conversion between baseband signals and bit streams according to a physical layer specification of a system. For example, upon data transmission, the baseband processor 1 i-20 may encipher and modulate a transmission bit stream to generate complex symbols. Also, upon data reception, the baseband processor 1 i-20 may demodulate and decipher a baseband signal provided from the RF processor 1 i-10 to restore a reception bit stream. For example, according to an OFDM method, upon data transmission, the baseband processor 1 i-20 may encipher and modulate a transmission bit stream to generate complex symbols, map the complex symbols to subcarriers, and then construct OFDM symbols through inverse fast Fourier transform (IFFT) and cyclic prefix (CP) insertion. Also, upon data reception, the baseband processor 1 i-20 may segment a baseband signal provided from the RF processor 1 i-10 in units of OFDM symbols, restore signals mapped to subcarriers through fast Fourier transform (FFT), and then restore a reception bit stream through demodulation and deciphering.

The baseband processor 1 i-20 and the RF processor 1 i-10 may transmit and receive signals as described above. Accordingly, the baseband processor 1 i-20 and the RF processor 1 i-10 are also referred to as a transmitter, a receiver, a transceiver, or a communicator. Furthermore, at least one of the baseband processor 1 i-20 or the RF processor 1 i-10 may include a plurality of communication modules to support a plurality of different radio access technologies. Also, at least one of the baseband processor 1 i-20 or the RF processor 1 i-10 may include different communication modules to process signals of different frequency bands. For example, the different radio access technologies may include a LET network, a NR network, etc. Also, the different frequency bands may include a super high frequency (SHF) (e.g., 2.1 GHz and 1 GHz) band and a millimeter wave (e.g., 60 GHz) band. The UE may transmit/receive a signal to/from a gNB by using the baseband processor 1 i-20 and the RF processor 1 i-10, wherein the signal may include control information and data.

The storage device 1 i-30 may store data, such as basic programs, application programs, and configuration information, for operations of the UE. The storage device 1 i-30 may provide the stored data according to a request from the controller 1 i-40. The storage device 1 i-30 may be configured with a storage medium, such as read only memory (ROM), random access memory (RAM), a hard disc, compact disc ROM (CD-ROM), digital versatile disc (DVD), and the like, or a combination of such a storage media. Also, the storage device 1 i-30 may be configured with a plurality of memories.

The controller 1 i-40 may control overall operations of the UE. For example, the controller 1 i-40 may transmit and receive signals through the baseband processor 1 i-20 and the RF processor 1 i-10. Also, the controller 1 i-40 may write data in the storage device 1 i-30 or read data from the storage device 1 i-30. To write or read data, the controller 1 i-40 may include at least one processor. For example, the controller 1 i-40 may include a communication processor CP for performing control for communication, and an application processor AP for controlling an upper layer of an application program, etc. Also, the controller 1 i-40 may include multiple connection processor 1 i-42 for processing multiple connections. Also, the controller 1 i-40 may control the UE to perform a method for performing the handover procedure described above. Also, at least one component of the UE may be implemented as a single chip.

FIG. 1J shows a block configuration of a Transmission/Reception Point (TRP) or a wireless node in a wireless communication system according to an embodiment of the disclosure.

Referring to FIG. 1J, a gNB (also, referred to as a TRP or a wireless node) may include a RF processor 1 j-10, a baseband processor 1 j-20, a backhaul communicator 1 j-30, a storage device 1 j-40, and a controller 1 j-50. However, a configuration of the gNB is not limited to the embodiment shown in FIG. 1J, and the gNB may include more or less components than those shown in FIG. 1J.

The RF processor 1 j-10 may perform a function for transmitting and receiving signals through a wireless channel, such as band conversion, amplification, etc. of signals. That is, the RF processor 1 j-10 may up-convert a baseband signal provided from the baseband processor 1 j-20 into a RF band signal, transmit the RF band signal through an antenna, and down-convert an RF band signal received through the antenna into a baseband signal. For example, the RF processor 1 j-10 may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc. In FIG. 1J, a single antenna is shown, however, the gNB may include a plurality of antennas. Also, the RF processor 1 j-10 may include a plurality of RF chains. Furthermore, the RF processor 1 j-10 may perform beamforming. For beamforming, the RF processor 1 j-10 may adjust phases and magnitudes of signals that are transmitted/received through the plurality of antennas or antenna elements. The RF processor 1 j-10 may perform a downlink MIMO operation by transmitting one or more layers.

The baseband processor 1 j-20 may perform a function of conversion between baseband signals and bit streams according to a physical layer specification of first radio access technology. For example, upon data transmission, the baseband processor 1 j-20 may encipher and modulate a transmission bit stream to generate complex symbols. Also, upon data reception, the baseband processor 1 j-20 may demodulate and decipher a baseband signal provided from the RF processor 1 j-10 to restore a reception bit stream. For example, according to the OFDM method, upon data transmission, the baseband processor 1 j-20 may encipher and modulate a transmission bit stream to generate complex symbols, map the complex symbols to subcarriers, and then construct OFDM symbols through IFFT and CP insertion. Also, upon data reception, the baseband processor 1 j-20 may segment a baseband signal provided from the RF processor 1 j-10 in units of OFDM symbols, restore signals mapped to subcarriers through FFT, and then restore a reception bit stream through demodulation and deciphering. The baseband processor 1 j-20 and the RF processor 1 j-10 may transmit and receive signals as described above. Accordingly, the baseband processor 1 j-20 and the RF processor 1 j-10 are also referred to as a transmitter, a receiver, a transceiver, a communicator, or a wireless communicator. The gNB may transmit/receive a signal to/from the UE by using the baseband processor 1 j-20 and the RF processor 1 j-10, wherein the signal may include control information and data.

The communicator 1 j-30 may provide an interface for communicating with other nodes in a network.

The storage device 1 j-40 may store data, such as basic programs, application programs, and configuration information, for operations of the gNB. Particularly, the storage device 1 j-40 may store information about a bearer assigned to a UE connected to the gNB, a measurement result reported from the connected UE, etc. Also, the storage device 1 j-40 may store information that is used as a determination criterion about whether to provide multiple connections to the UE or whether to release multiple connections to the UE. Also, the storage device 1 j-40 may provide the stored data according to a request from the controller 1 j-50. The storage device 1 j-40 may be configured with a storage medium, such as ROM, RAM, a hard disc, CD-ROM, DVD, and the like, or a combination of such a storage media. Also, the storage device 1 j-40 may be configured with a plurality of memories.

The controller 1 j-50 may control overall operations of the gNB. For example, the controller 1 j-50 may transmit and receive signals through the baseband processor 1 j-20 and the RF processor 1 j-10 or through the backhaul communicator 1 j-30. Also, the controller 1 j-50 may write data in the storage device 1 j-40 or read data from the storage device 1 j-40. To write or read data, the controller 1 i-50 may include at least one processor. Also, the controller 1 j-50 may include multiple connection processor 1 j-52 for processing multiple connections. For example, the controller 1 i-50 may control the gNB such that the UE may perform a method for performing an RRC connection resume procedure as described above. Also, at least one component of the gNB may be implemented as a single chip.

Hereinafter, according to the disclosure, an indicator may be proposed to set an in-order delivery function for reception operations of a reception RLC layer of a next-generation mobile communication system with an RRC message, and when the in-order delivery function is set in the RLC layer with the indicator, detailed reception window operations of the RLC layer are proposed for the RLC UM mode or the RLC AM mode. According to an embodiment of the disclosure, data may be transferred in order to the PDCP layer. Also, data loss that may be generated in the PDCP layer may be prevented, and also, data processing that may be generated upon a handover may be simplified.

When a UE and a gNB transmit and receive data in a next-generation mobile communication system, a reception RLC layer may always perform an out-of-order delivery function. The out-of-order delivery function means a function of reassembling data received by the reception RLC layer in arrival order, without ordering the data in order of RLC SNs, and transferring a result of the reassembling to an upper layer. The out-of-order delivery function may be effective in view of transmission delay, but may cause data loss and increase complexity of handover processing because the PDCP layer drives a timer and moves a window when the timer is expired.

Hereinafter, a method and apparatus for processing communication data will be described with reference to FIGS. 2A to 2M.

FIG. 2A shows a structure of an LTE system to which an embodiment of the disclosure may be applied.

Referring to FIG. 2A, a radio access network of an LTE system may be configured with next-generation Evolved Node B (ENBs) (hereinafter, also referred to as Nodes B or base stations), 2 a-05, 2 a-10, 2 a-15, and 2 a-20, a MME 2 a-25, and a S-GW 2 a-30. A UE 2 a-35 may be connected to an external network through the ENBs 2 a-05 to 2 a-20 and the S-GW 2 a-30.

In FIG. 2A, the ENBs 2 a-05 to 2 a-20 may correspond to existing nodes B of a UMTS. The ENBs 2 a-05 to 2 a-20 may be connected to the UE 2 a-35 through a wireless channel, and perform more complicated functions than the existing nodes B. In the LTE system, because all user traffics including a real-time service such as VoIP are serviced through a shared channel, a device for performing scheduling by collecting status information of UEs, such as buffer statuses of UEs, available transmission power states of UEs, channel states of UEs, etc., is needed. The ENBs 2 a-05 to 2 a-20 may function as such a device. An ENB may generally control a plurality of cells. For example, to implement a transmission speed of 100 Mbps, the LTE system uses, as radio access technology, OFDM, for example, in a bandwidth of 20 MHz. Also, AMC of determining a modulation scheme and a channel coding rate according to a channel state of a UE may be applied. The S-GW 2 a-30, which is a device for providing data bearers, may generate or remove data bearers according to a control of the MME 2 a-25. The MME 2 a-25 may be in charge of various control functions, as well as a mobility management function for UEs, and may be connected to a plurality of ENB s.

FIG. 2B shows a radio protocol architecture of an LTE system to which an embodiment of the disclosure may be applied.

Referring to FIG. 2B, radio protocols of the LTE system may include PDCPs 2 b-05 and 2 b-40, RLC 2 b-10 and 2 b-35, and MAC 2 b-15 and 2 b-30 of a UE and an ENB. The PDCPs 2 b-05 and 2 b-40 may be in charge of operations of IP header compression/restoration. Main functions of the PDCPs 2 b-05 and 2 b-40 may be summarized as follows.

-   -   Header compression and decompression: ROHC only     -   Transfer of user data     -   In-sequence delivery of upper layer PDUs at PDCP         re-establishment procedure for RLC AM     -   For split bearers in DC (only support for RLC AM): PDCP PDU         routing for transmission and PDCP PDU reordering for reception     -   Duplicate detection of lower layer SDUs at PDCP re-establishment         procedure for RLC AM     -   Retransmission of PDCP SDUs at handover and, for split bearers         in DC, of PDCP PDUs at PDCP data-recovery procedure, for RLC AM     -   Ciphering and deciphering     -   Timer-based SDU discard in uplink

The RLC 2 b-10 and 2 b-35 may reconstruct a PDCP PDU to an appropriate size to perform an ARQ operation, etc. Main functions of the RLC 2 b-10 and 2 b-35 may be summarized as follows.

-   -   Transfer of upper layer PDUs     -   Error Correction through ARQ (only for AM data transfer)     -   Concatenation, segmentation and reassembly of RLC SDUs (only for         UM and AM data transfer)     -   Re-segmentation of RLC data PDUs (only for AM data transfer)     -   Reordering of RLC data PDUs (only for UM and AM data transfer)     -   Duplicate detection (only for UM and AM data transfer)     -   Protocol error detection (only for AM data transfer)     -   RLC SDU discard (only for UM and AM data transfer)     -   RLC re-establishment

The MAC 2 b-15 and 2 b-30 may be connected to several RLC layers configured in the UE, and perform an operation of multiplexing RLC PDUs into a MAC PDU and demultiplexing RLC PDUs from a MAC PDU. Main functions of the MAC 2 b-15 and 2 b-30 may be summarized as follows.

-   -   Mapping between logical channels and transport channels     -   Multiplexing/demultiplexing of MAC SDUs belonging to one or         different logical channels into/from transport blocks (TB)         delivered to/from the physical layer on transport channels     -   Scheduling information reporting     -   Error correction through HARQ     -   Priority handling between logical channels of one UE     -   Priority handling between UEs by means of dynamic scheduling     -   MBMS service identification     -   Transport format selection     -   Padding

Physical layers (PHY) 2 b-20 and 2 b-25 may perform an operation of performing channel coding and modulation on upper layer data to generate an OFDM symbol and transmitting the OFDM symbol through a wireless channel, or demodulating an OFDM symbol received through the wireless channel, performing channel decoding on the OFDM symbol, and transferring the channel-decoded OFDM symbol to an upper layer.

FIG. 2C shows a structure of a next-generation mobile communication system to which an embodiment of the disclosure may be applied.

Referring to FIG. 2C, a radio access network of a next-generation mobile communication system (hereinafter, also referred to as NR or 5G) may be configured with a next-generation NR gNB (also, referred to as a NR base station) 2 c-10 and a NR CR 2 c-05. A NR UE (also, referred to as a UE) 2 c-15 may be connected to an external network through the NR gNB 2 c-10 and the NR CN 2 c-05.

In FIG. 2C, the NR gNB 2 c-10 may correspond to an eNB of an existing LTE system. The NR gNB 2 c-10 may be connected to the NR UE 2 c-15 through a wireless channel, and provide a more excellent service than an existing node B. In a next-generation mobile communication system, because all user traffics are serviced through a shared channel, a device for performing scheduling by collecting status information of UEs, such as buffer statuses of UEs, available transmission power states of UEs, channel states of UEs, etc., is needed. The NR gNB 2 c-10 may function as such a device. A NR gNB may generally control a plurality of cells. To implement ultra high-speed data transmission compared to existing LTE, OFDM as radio access technology may be combined with beam-forming technology, while providing an existing maximum bandwidth or more.

Also, AMC of determining a modulation scheme and a channel coding rate according to a channel state of a UE may be applied. The NR CN 2 c-05 may perform functions, such as mobility support, bearer setup, Quality of Service (QoS) setup, etc. The NR CN 2 c-05 may be in charge of various control functions, as well as a mobility management function for UEs, and may be connected to a plurality of NR gNBs. Also, the next-generation mobile communication system may interwork with an existing LTE system, and the NR CN 2 c-05 may be connected to the MME 2 c-25 through a network interface. The MME 1 c-25 may be connected to an eNB 2 c-30 which is an existing base station.

FIG. 2D shows a radio protocol architecture of a next-generation mobile communication system to which an embodiment of the disclosure may be applied.

Referring to FIG. 2D, radio protocols of the next-generation mobile communication system may include NR SDAPs 2 d-01 and 2 d-45, NR PDCPs 2 d-05 and 2 d-40, NR RLC 2 d-10 and 2 d-35, and NR MAC 2 d-15 and 2 d-30 of an UE and a NR gNB.

Main functions of the NR SDAPs 2 d-01 and 2 d-45 may include some of the following functions.

-   -   Transfer of user plane data     -   Mapping between a QoS flow and a DRB for both DL and UL     -   Marking QoS flow ID in both DL and UL packets     -   Reflective QoS flow to DRB mapping for the UL SDAP PDUs

In regard of a SDAP layer as described above, the UE may receive, as an RRC message, a setting about whether to use a header of the SDAP layer or whether to use a function of the SDAP layer for each PDCP layer, for each bearer, or for each logical channel. When a SDAP header is set, the SDAP header capable of indicating the UE to update or reset mapping information about a QoS flow and a data bearer of a uplink and a downlink with a 1-bit indicator NAS reflective QoS for a NAS reflective QoS setting and a 1-bit indicator AS reflective QoS for an AS reflective QoS setting of the SDAP header may include QoS flow ID information representing QoS. QoS information may be used as data processing priority, scheduling information, etc. for supporting a stable service.

Main functions of the NR PDCPs 2 d-05 and 2 d-40 may include some of the following functions.

-   -   Header compression and decompression: ROHC only     -   Transfer of user data     -   In-sequence delivery of upper layer PDUs     -   Out-of-sequence delivery of upper layer PDUs     -   PDCP PDU reordering for reception     -   Duplicate detection of lower layer SDUs     -   Retransmission of PDCP SDUs     -   Ciphering and deciphering     -   Timer-based SDU discard in uplink

The reordering function of a NR PDCP device among the above-mentioned functions means a function of reordering PDCP PDUs received from a lower layer in order based on PDCP SNs, and may include a function of transferring data to an upper layer in the reordered order. Alternatively, the reordering function of the NR PDCP device may include a function of directly transferring data regardless of order, a function of recording lost PDCP PDUs by reordering, a function of sending a status report for lost PDCP PDUs to a transmitter, and a function of requesting retransmission of lost PDCP PDUs.

Main functions of the NR RLC 2 d-10 and 2 d-35 may include some of the following functions.

-   -   Transfer of upper layer PDUs     -   In-sequence delivery of upper layer PDUs     -   Out-of-sequence delivery of upper layer PDUs     -   Error Correction through ARQ     -   Concatenation, segmentation and reassembly of RLC SDUs     -   Re-segmentation of RLC data PDUs     -   Reordering of RLC data PDUs     -   Duplicate detection     -   Protocol error detection     -   RLC SDU discard     -   RLC re-establishment

The in-sequence delivery function of a NR RLC device among the above-mentioned functions means a function of transferring RLC SDUs received from a lower layer in order to an upper layer, and may include a function of reassembling, when a RLC SDU is segmented into several RLC SDUs and received, the RLC SDUs and transferring a result of the reassembling, a function of reordering received RLC PDUs based on RLC SNs or PDCP SNs, a function of reordering and recording lost RLC PDUs, a function of sending a status report for lost RLC PDUs to a transmitter, a function of requesting retransmission of lost RLC PDUs, a function of transferring, when there are lost RLC SDUs, RLC SDUs before the lost RLC SDUs in order to the upper layer, a function of transferring, when a preset timer has been expired although there are lost RLC SDUs, all RLC SDUs received before the preset timer starts in order to the upper layer, or a function of transferring, when the preset time has been expired although there are lost RLC SDUs, all RLC SDUs received up to now in order to the upper layer.

Also, RLC PDUs may be processed in order (in arrival order, regardless of SNs or order of SNs) in which they are received, and transferred to the PDCP device out of sequence delivery, and in the case of segments, segments which have been stored in a buffer or which are to be received later may be received and reconstructed into a compete RLC PDU and then processed and transferred to the PDCP device. A NR RLC layer as described above may include no concatenation function, and the above-described function may be performed in a NR MAC layer or replaced with a multiplexing function of the NR MAC layer.

The out-of-sequence delivery function of the NR RLC device as described above means a function of transferring RLC SDUs received from a lower layer to the upper layer regardless of order, and may include a function of reassembling, when a RLC SDU is segmented into several RLC SDUs and received, the RLC SDUs and transferring a result of the reassembling, and a function of storing and ordering RLC SNs or PDCP SNs of the received RLC PDUs and recording lost RLC PDUs.

The NR MAC 2 d-15 and 2 d-30 may be connected to several NR RLC layers configured in the UE, and main functions of the NR MAC 2 d-15 and 2 d-30 may include some of the following functions.

-   -   Mapping between logical channels and transport channels     -   Multiplexing/demultiplexing of MAC SDUs     -   Scheduling information reporting     -   Error correction through HARQ     -   Priority handling between logical channels of one UE     -   Priority handling between UEs by means of dynamic scheduling     -   MBMS service identification     -   Transport format selection     -   Padding

NR PHY layers 2 d-20 and 2 d-25 may perform an operation of performing channel coding and modulation on upper layer data to generate an OFDM symbol and transferring the OFDM symbol through a wireless channel, or performing demodulation and channel decoding on an OFDM symbol received through the wireless channel and transferring a result of the modulation and channel decoding to an upper layer.

FIG. 2E is a view for describing a procedure in which a gNB according to an embodiment of the disclosure releases a connection of a UE for the UE to change from the RRC connected mode to the RRC idle mode and a procedure in which the UE sets up a connection to the gNB to change from the RRC idle mode to the RRC connected mode.

Referring to FIG. 2E, when the UE transmitting and receiving data in the RRC connected mode receives or transmits no data for a certain reason or for a preset time, the gNB may transmit an RRCConnectionRelease message to the UE to change the UE to the RRC idle mode, in operation 2 e-01. The UE (hereinafter, also referred to as an idle mode UE) in which no connection is currently established may perform, when data to be transmitted is generated, an RRC connection establishment process together with the gNB. The UE may establish reverse transmission synchronization with the gNB through a random access process, and transmit an RRCConnectionRequest message to the gNB, in operation 2 e-05. The RRCConnectionRequest message may include an identifier of the UE and a cause establishmentCause for connection establishment.

The gNB may transmit an RRCConnectionSetup message to the UE to enable the UE to establish an RRC connection, in operation 2 e-10. The RRCConnectionSetup message may include RRC connection configuration information, etc. Also, the RRCConnectionSetup message may include at least one information of bearer configuration information of the UE, configuration information of a PHY layer, configuration information of a MAC layer, configuration information of a RLC layer, configuration information of a PDCP layer, or configuration information of a SDAP layer.

The configuration information of the RLC layer may include an indicator defined to indicate whether to set the RLC layer to the RLC TM mode, the RLC UM mode, or the RLC AM mode and whether to set or activate the in-order delivery function for the RLC layer. The gNB may indicate the UE of a setting about an RLC mode and whether or not the in-order delivery function is set for each bearer or for each RLC layer, based on the configuration information of the RLC layer. When an indicator for setting or activating the in-order delivery function is included together with configuration information about the RLC mode in the configuration information of the RLC layer, the UE may set the RLC mode (for example, RLC TM, RLC UM or RLC AM) for the reception RLC layer, and perform an operation of the reception RLC layer to which the in-order delivery function is applied. When no indicator for setting or activating the in-order delivery function for the RLC layer is included in the configuration information of the RLC layer, the UE may perform an operation of the reception RLC layer to which the out-of-order delivery function is basically applied.

An RRC connection is also referred to as a Signaling Radio Bearer (SRB), and may be used to transmit and receive an RRC message which is a control message between a UE and a gNB. The UE which has established the RRC connection may transmit an RRCConnetionSetupComplete message to the gNB, in operation 2 e-15. The RRCConnetionSetupComplete message may include a SERVICE REQUEST message as a control message through which the UE requests the MME to set a bearer for a preset service.

The gNB may transmit the SERVICE REQUEST message included in the RRCConnetionSetupComplete message to an Access and Mobility Management Function (AMF), in operation 2 e-20, and the AMF may determine whether to provide the preset service requested by the UE. When the AMF determines to provide the preset service requested by the UE, the AMF may transmit an INITIAL CONTEXT SETUP REQUEST message to the gNB, in operation 2 e-25. The INITIAL CONTEXT SETUP REQUEST message may include QoS information to be applied for a Data Radio Bearer (DRB) setting, security-related information (for example, a security key or a security algorithm) to be applied to the DRB, etc.

The gNB may exchange a SecurityModeCommand message with a SecurityModeComplete message to set security with the UE, in operations 2 e-30 and 2 e-35. When a security setting is completed, the gNB may transmit an RRCConnectionReconfiguration message to the UE, in operation 2 e-40.

Also, the RRCConnectionReconfiguration message may include bearer configuration information of the UE, configuration information of the PHY layer, configuration information of the MAC layer, configuration information of the RLC layer, configuration information of the PDCP layer, or configuration information of the SDAP layer.

The configuration information of the RLC layer may include an indicator defined to indicate whether to set the RLC layer to the RLC TM mode, the RLC UM mode, or the RLC AM mode and whether to set or activate the in-order delivery function for the RLC layer. The gNB may indicate the UE of a setting about the RLC mode and whether or not the in-order delivery function is set for each bearer or for each RLC layer, based on the configuration information of the RLC layer. When an indicator for setting or activating the in-order delivery function is included together with configuration information about the RLC mode in the configuration information of the RLC layer, the UE may set the RLC mode (for example, RLC TM, RLC UM or RLC AM) for the reception RLC layer, and perform an operation of the reception RLC layer to which the in-order delivery function is applied. When no indicator for setting or activating the in-order delivery function for the RLC layer is included in the configuration information of the RLC layer, the UE may perform an operation of the reception RLC layer to which the out-of-order delivery function is basically applied.

The RRCConnectionReconfiguration message may include configuration information of a DRB by which user data is to be processed, and the UE may apply the configuration information of the DRB to set up the DRB, and transmit an RRCConnectionReconfigurationComplete message to the gNB, in operation 2 e-45.

After the gNB completes DRB setup with the UE, the gNB may transmit an initial context setup complete message to the AMF, in operation 2 e-50. The AMF may receive the initial context setup complete message, and perform a session management procedure with a User Plane Function (UPF) to establish a PDU session, in operation 2 e-60. When the above-described process is completed, the UE may transmit/receive data to/from the gNB through the UPF, in operations 2 e-65 and 2 e-70.

As described above, a general data transmission process may be configured with three operations of RCC connection setup, security setup, and DRB setup. Also, the gNB may transmit an RRCConnectionReconfiguration message to newly set up, add, or change a setting to the UE for a certain reason, in operation 2 e-75.

According to an embodiment of the disclosure, the bearer may have a meaning including a SRB and a DRB, wherein the SRB means a Signaling Radio Bearer and the DRB means a Data Radio Bearer. The SRB may be mainly used to transmit and receive RRC messages of the RRC layer, and the DRB may be mainly used to transmit and receive user layer data. Also, UM DRB means a DRB using the RLC layer operating in an Unacknowledged Mode (UM), and AM DRB means a DRB using the RLC layer operating in an Acknowledged Mode (AM).

FIG. 2F shows a handover procedure in a next-generation mobile communication system according to an embodiment of the disclosure.

Referring to FIG. 2F, a UE 2 f-01 which is in the RRC connected mode may send a cell measurement report to a source gNB 2 f-02 periodically or when a preset event is satisfied, in operation 2 f-05. The source gNB 2 f-02 may determine whether to perform a handover of the UE 2 f-01 to an adjacent cell based on the cell measurement information. The handover may be a technique of changing a source gNB providing a service to a UE being in a connected mode to another gNB (or another cell in the same gNB).

When the source gNB 2 f-02 determines a handover, the source gNB 2 f-02 may send a Handover (HO) request message to a new gNB, that is, a target gNB 2 f-03 that is to provide a service to the UE 2 f-01, thereby requesting a handover, in operation 2 f-10. When the target gNB 2 f-03 accepts the handover request, the target gNB 2 f-03 may transmit a HO request Ack message to the source gNB 2 f-02, in operation 2 f-15. When the source gNB 2 f-02 receives the HO request Ack message, the source gNB 2 f-02 may transmit a HO command message to the UE 2 f-01, in operation 2 f-20. The source gNB 2 f-02 may transfer the HO command message to the UE 2 f-01 by using an RRC Connection Reconfiguration message, in operation 2 f-20. When the UE 2 f-01 receives the HO command message, the UE 2 f-01 may stop data transmission/reception to/from the source gNB 2 f-02, and start a T304 timer. The T304 timer may return the UE 2 f-01 to its original setting and change the UE 2 f-01 to the RRC idle state, when the UE 2 f-01 fails to perform a handover to the target gNB 2 f-03 within a preset time period. The source gNB 2 f-02 may transfer a SN status about a transmission/reception status of uplink/downlink data to the target gNB 2 f-03, and when there is downlink data, the source gNB 2 f-02 may transfer the downlink data to the target gNB 2 f-03, in operations 2 f-30 and 2 f-35. The UE 2 f-01 may attempt a random access to a target cell indicated from the source gNB 2 f-02, in operation 2 f-40. The random access may be aimed to notify the target cell that the UE 2 f-01 moves through a handover, while synchronizing an uplink. For the random access, the UE 2 f-01 may transmit a preamble corresponding to a preamble ID received from the source gNB 2 f-02 or a randomly selected preamble ID to the target cell. When a preset number of subframes elapse after the preamble is transmitted, the UE 2 f-01 may monitor whether a Random Access Response (RAR) message is transmitted from the target cell. A time period in which the UE 2 f-01 monitors whether the RAR message is transmitted is referred to as a RAR window. When a RAR message is received within the time period of the monitoring in operation 2 f-45, the UE 2 f-01 may transmit a HO complete message as an RRC Reconfiguration Complete message to the target gNB 2 f-03, in operation 2 f-55. When a random access response is successfully received from the target gNB 2 f-03, the UE 2 f-01 may stop the T304 timer, in operation 2 f-50.

To correct paths of bearers set to the source gNB 2 f-02, the target gNB 2 f-03 may request a core network 2 f-04 to correct paths in operations 2 f-60 and 2 f-65, and notify the source gNB 2 f-02 of discarding UE context of the UE 2 f-01, in operation 2 f-70. Accordingly, the UE 2 f-01 may attempt to receive data with respect to the target gNB 2 f-03 from when the RAR window starts, and after the RAR message is received, the UE 2 f-01 may transmit the RRCConnectionReconfigurationComplete message to the target gNB 2 f-03 and start data transmission/reception to/from the target gNB 2 f-03. According to an embodiment of the disclosure, the core network 2 f-04 may include at least one of an MME, a S-GW, or an AMF, and further include at least one of a UPF or a SMF.

In the above-described handover procedure, when the source gNB 2 f-02 transmits the HO command message (for example, an RRC Reconfiguration message) to the UE 2 f-01, at least one of information about a target gNB to which the UE 2 f-01 needs to perform a handover, information about a random access transmission resource for a cell, preamble information, bearer configuration information, or cell information may be set in the HO command message.

Also, the HO command message may include an indicator defined to indicate what kind of handover the UE 2 f-01 needs to perform. For example, the source gNB 2 f-01 may define indicators for a plurality of handover methods, and set one of the defined indicators such that the UE 2 f-01 perform a handover method corresponding to the set indicator. The plurality of handover methods may include at least one method of a RACH-less handover method, an MBB handover method, an eMBB method, or a conditional handover method. Also, the HO command message may indicate whether to set or activate the in-order delivery function for each RLC layer of the UE 2 f-01 through the indicator.

In an operation of a protocol layer of the next-generation mobile communication system described above with reference to FIG. 2D, an issue that data loss is generated in the PDCP layer due to the out-of-order delivery of the RLC layer may be generated.

For example, the reception PDCP layer may drive a PDCP reordering timer. The PDCP reordering timer may be driven or start when a PDCP SN gap is generated in the reception PDCP layer based on PDCP SNs, and the reception PDCP layer may transfer data to an upper layer in ascending order of the PDCP SNs or COUNT values when data corresponding to the PDCP SN gap does not arrive until the PDCP reordering timer is expired, and move after a PDCP SN transferring a reception window. Accordingly, when data corresponding to the PDCP SN gap arrives late after the PDCP reordering timer is expired, the arrived data may not be data in the reception window, and therefore, the reception PDCP layer may discard the data, resulting in data loss. A reason why data arrives late may be because several retransmissions of the RLC layer set to the RLC AM mode. However, a reason why the PDCP reordering timer is triggered may be because the reception RLC layer transfers data quickly to the upper layer through the out-of-order delivery function.

According to an embodiment of the disclosure, when AM DRB is set for a UE with an RRC message, a gNB may set the in-order delivery function with respect to a reception RLC layer for the UE. When long retransmission of the RLC layer is required or when data loss is fatal to a service, the gNB may set or activate the in-order delivery function with respect to the reception RLC layer.

Also, as shown in FIG. 2F, in the case that a handover method for reducing a data interruption time generated in a handover is indicated in a handover situation, for example, when a UE performs a handover while continuously transmitting/receiving data to/from a source gNB and simultaneously transmitting/receiving data to/from a target gNB to prevent any data transmission/reception interruption, each RLC layer may perform out-of-order delivery of data received from different gNBs when a reception PDCP layer of the UE needs to decipher downlink data with different security keys for the different gNBs or needs to perform a compression release procedure for releasing header compression in order. In this case, the reception PDCP layer may have high complexity in processing the out-of-order data. Accordingly, when a preset handover method is indicated in a HO command message, the gNB may cause the reception RLC layer of the UE to perform the in-order delivery function, or may indicate the RLC layer to perform the in-order delivery function together with an indicator indicating the preset handover method through the indicator. The indication for setting, activating or deactivating the in-order delivery function may be dynamically set by a transmitter (gNB or UE) by defining a new RLC control PDU instead of an RRC message.

After performing the handover, the UE may itself fall back to the out-of-order delivery function from the in-order delivery function. For example, after the UE transmits a handover completion message to the target gNB or when the UE drives a preset timer and then the preset timer is expired, the UE may fall back to the out-of-order delivery function from the in-order delivery function. A value of the preset timer may be indicated by the gNB through a handover command message. Alternatively, when the gNB indicates the out-of-order delivery function through an RRC message or a new RLC control PDU while the reception RLC layer of the UE continues to apply an in-order delivery function even after a handover is performed, the UE may change from the in-order delivery function to the out-of-order delivery function.

Hereinafter, a first embodiment for operations of the reception RLC layer proposed in the disclosure will be described in detail and proposed in regard of the RLC UM mode and the RLC AM mode.

In the first embodiment for operations of the reception RLC layer proposed in an embodiment of the disclosure, the out-of-order delivery function may be performed as a basic operation.

First, variables used for a window operation of the reception RLC layer in the RLC UM mode may be defined as follows.

-   -   RX_Next_Reassembly: state variable indicating a lowest or         earliest RLC SN among data (RLC PDU or RLC SDU) considered to be         reassembled in the reception RLC layer     -   RX_Timer_Trigger: state variable indicating an RLC SN following         an RLC SN triggering an RLC reassembly timer     -   RX_Next_Highest: state variable indicating an RLC SN following a         highest RLC SN among received data

Detailed operations of a 1-1 embodiment when the reception RLC layer according to an embodiment of the disclosure operates in the RLC UM mode may be as follows.

The reception RLC layer may operate, when receiving a UMD PDU from a lower layer, as follows.

-   -   When no RLC SN is included in a header of the received UMD PDU,     -   Remove an RLC header and transfer an RLC SDU to an upper layer.     -   When an RLC SN is included in the header of the received UMD PDU         and the RLC SN is equal to or greater than a         RX_Next_Highest−UM_Window_Size value and smaller than a         RX_Next_Reassembly value,     -   Discard the received UMD PDU.     -   When an RLC SN is included in the header of the received UMD PDU         and the RLC SN is smaller than the         RX_Next_Highest−UM_Window_Size value and equal to or greater         than the RX_Next_Reassembly value,     -   Store the received UMD PDU in a reception buffer.

The reception RLC layer may operate in regard of the UMD PDU stored in the buffer in the above-described procedure, as follows. That is, when a UMD PDU having an RLC SN of a x value is stored in the buffer, the reception RLC layer may operate as follows.

-   -   When all segments having the RLC SN of the x value are received,     -   Reassemble all the segments having the RLC SN of the x value to         configure an RLC SDU, remove an RLC header, and then transfer         the RLC SDU to the upper layer.     -   When the x value is equal to the RX_Next_Reassembly value,     -   Update a RX_Next_Reassembly variable to a RLC SN of first data         (or data having a lowest RLC SN among data having great RLC SNs)         having a RLC SN that is greater than the current         RX_Next_Reassembly value among data neither yet reassembled nor         transferred to the upper layer.     -   When the x value of the RLC SN is out of an RLC reassembly         window,     -   Update a RX_Next_Highest variable to x+1.     -   Discard all data corresponding to RLC SNs being out of the RLC         reassembly window.     -   When the RX_Next_Reassembly value is out of the RLC reassembly         window,     -   Set the RX_Next_Reassembly variable to the RLC SN of the first         data (or data having the lowest RLC SN among the data having the         great RLC SNs) having a RLC SN that is equal to or greater than         the current RX_Next_Highest−UM_Window_Size value among the data         neither yet reassembled nor transferred to the upper layer.     -   When the RLC reassembly timer is being driven,     -   When a RX_Timer_Trigger value is equal to or smaller than the         RX_Next_Reassembly value,     -   Or, when the RX_Timer_Trigger value is out of the RLC reassembly         window and the RX_Timer_Trigger value is not equal to the         RX_Next_Highest value,     -   Or, when the RX_Next_Highest value is equal to a         RX_Next_Reassembly+1 value and there is no segment lost before a         final byte of all segments currently received for a RLC SDU         having the RX_Next_Reassembly value as a RLC SN,     -   Stop and initialize the RLC reassembly timer.     -   When the RLC reassembly timer is not driven (including a case in         which the RLC reassembly timer stops according to a procedure),     -   When the RX_Next_Highest value is greater than the         RX_Next_Reassembly+1 value,     -   Or, when the RX_Next_Highest value is equal to the         RX_Next_Reassembly+1 value and there is at least one segment         lost before a final byte of all segments currently received for         a RLC SDU having the RX_Next_Reassembly value as a RLC SN,     -   Start the RLC reassembly timer, and     -   Set the RX_Timer_Trigger variable to the RX_Next_Highest value.

When the RLC reassembly timer is expired in the above-described procedure, the reception RLC layer may operate as follows.

-   -   Update the RX_Next_Reassembly variable to a RLC SN of first data         (or data having a lowest RLC SN) having a RLC SN that is equal         to or greater than the RX_Timer_Trigger value, the first data         not yet reassembled.     -   Discard all segments having an RLC SN that is smaller than the         updated RX_Next_Reassembly value.     -   When the RX_Next_Highest value is greater than the         RX_Next_Reassumbly+1 value,     -   Or, when the RX_Next_Highest value is equal to the         RX_Next_Reassumbly+1 value and there is at least one segment         lost before a final byte of all segments currently received for         a RLC SDU having the RX_Next_Reassembly value as a RLC SN,     -   Start the RLC reassembly timer, and     -   Set the RX_Timer_Trigger variable to the RX_Next_Highest value.

FIG. 2G shows RLC header formats that may be applied in the 1-1 embodiment which are operations of the reception RLC layer in which the RLC UM mode according to an embodiment of the disclosure is set.

Referring to FIG. 2G, in the 1-1 embodiment which is an operation of the reception RLC layer in which the RLC UM mode according to an embodiment of the disclosure is set, when a transmission RLC layer transmits data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU, the transmission RLC layer may apply a RLC header 2 g-05 including a segmentation info (SI) field (indicating the complete data, a first segment, a final segment, or a middle segment being neither the first segment nor the final segment) without a RLC SN to the data and transmit the data, in the case that a 6-bit RLC SN has been set in the RLC layer. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 g-10 including a RLC SN and a SI field to the first segment and transmit the first segment, and apply a RLC header 2 g-20 including a RLC SN, a SI field, and a segment offset (SO) field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

Also, when the transmission RLC layer transmits a RLC SDU as complete data without separating the RLC SDU, the transmission RLC layer may apply the RLC header 2 g-05 including a SI field to the data without a RLC SN and transmit the data, in the case in which a 12-bit RLC SN has been set in the RLC layer. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 g-15 including a RLC SN and a SI field to the first segment data and transmit the first segment, and apply a RLC header 2 g-25 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

Detailed operations of a 1-2 embodiment when the reception RLC layer according to an embodiment of the disclosure operates in the RLC AM mode may be as follows. In the RLC AM mode, the RLC layer may implement an ARQ function, receive an RLC status report (indicating successful data transfer with an RLC SN) from a receiving terminal, and retransmit data not successfully transmitted, thereby preventing data loss.

First, in the RLC AM mode, variables that are used for a window operation of the reception RLC layer may be defined as follows.

-   -   RX_Next: status variable indicating an RLC SN following a         highest or final RLC SN among completely received data which is         capable of being reassembled     -   RX_Next_Status_Trigger: status variable indicating an RLC SN         following an RLC SN triggering the RLC assembly timer     -   RX_Next_Highest: status variable indicating an RLC SN following         a highest RLC SN among received data     -   RX_Next_Highest_Status: status variable indicating a highest RLC         SN (for example, an RLC SN following a highest RLC SN         successfully received) that may be indicated as ACK_SN in an RLC         status report when the RLC status report is configured

When the reception RLC layer receives an AMD PDU including y to z bytes of an RLC SDU having an RLC SN of a x value from a lower layer, the reception RLC layer may operate as follows.

-   -   When the RLC SN of the x value of the received AMD PDU is out of         an RLC reception window,     -   Or, when an AMD PDU including y to z bytes of an RLC SDU having         the RLC SN of the x value has been previously received,     -   Discard the received AMD PDU.     -   Otherwise,     -   Store the received AMD PDU in the reception buffer.     -   When a portion or segment included in the AMD PDU has been         previously received,     -   Consider that the portion or segment included in the AMD PDU has         been received in duplicate and discard the portion or segment.

The reception RLC layer may operate in regard of the AMD PDU stored in the reception buffer in the procedure, as follows. That is, when the AMD PDU having the RLC SN of the x value is stored in the reception buffer, the RLC layer may operate as follows.

-   -   When the RLC SN of the x value is equal to or greater than a         RX_Next_Highest value,     -   Update the RX_Next_Highest value to x+1     -   When all bytes of the RLC SDU corresponding to the RLC SN of the         x value are received,     -   Reassemble data or segments corresponding to the RLC SN of the x         value to configure an RLC SDU, remove an RLC header, and then         transfer the reassembled RLC SDU to an upper layer.     -   When the x value is equal to a RX_Highest_Status value,     -   Update a RX_Highest_Status variable to an RLC SN of first data         (having a lowest RLC SN) which is greater than the         RX_Highest_Status value and of which all bytes are not yet         completely received.     -   When the x value is equal to a RX_Next value,     -   update a RX_Next variable to an RLC SN of first data (having a         lowest RLC SN) which is greater than the RX_Next value and of         which all bytes are not yet completely received.     -   When the RLC reassembly timer is being driven,     -   When a RX_Next_Status_Trigger value is equal to the RX_Next         value,     -   Or, when the RX_Next_Status_Trigger value is equal to a         RX_Next+1 value and there is no segment lost before a final byte         of all segments currently received for a RLC SDU having the         RX_Next value as a RLC SN,     -   Or, when the RX_Next_Status_Trigger value is out of an RLC         reception window and the RX_Next_Status_Trigger value is not         equal to a RX_Next+AM_Window_Size value,     -   Stop and initialize the RLC reassembly timer.     -   When the RLC reassembly timer is not driven (including a case in         which the RLC reassembly timer stops according to a procedure),     -   When the RX_Next_Highest value is greater than the RX_Next+1         value,     -   When the RX_Next_Highest value is equal to the RX_Next+1 value         and there is at least one segment lost before a final byte of         all segments currently received for an RLC SDU having the         RX_Next value as an RLC SN,     -   Start the RLC reassembly timer, and     -   Set the RX_Next_Status_Trigger variable to the RX_Next_Highest         value.

When the RLC reassembly timer is expired in the procedure, the reception RLC layer may operate as follows.

-   -   Update the RX_Highest_Status variable to an RLC SN of first data         (or data having a lowest RLC SN) not yet completely received,         the RLSN SN being equal to or greater than the         RX_Next_Status_Trigger value.     -   When the RX_Next_Highest value is greater than the RX_Next+1         value,     -   When the RX_Next_Highest value is equal to the RX_Next+1 value         and there is at least one segment lost before a final byte of         all segments currently received for an RLC SDU having the         RX_Next value as an RLC SN,     -   Start the RLC reassembly timer     -   Set the RX_Next_Status_Trigger variable to the RX_Next_Highest         value.

FIG. 2H shows RLC header formats that may be applied in a 1-2 embodiment which are operations of the reception RLC layer in which the RLC AM mode according to an embodiment of the disclosure is set.

Referring to FIG. 2H, in the 1-2 embodiment which are the operations of the reception RLC layer in which the RLC AM mode according to an embodiment of the disclosure is set, when the transmission RLC layer transmits data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU or when the transmission RLC layer transmits a first segment segmented from the RLC SDU, the transmission RLC layer may apply a RLC header 2 h-05 including a RLC SN and a SI field (indicating the complete data, the first segment, a final segment, or a middle segment being neither the first segment nor the final segment) to the data and transmit the data, in the case that a 12-bit RLC SN has been set in the RLC layer. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 h-15 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

Also, in the 1-2 embodiment which are the operations of the reception RLC layer in which the RLC AM mode according to an embodiment of the disclosure is set, when the transmission RLC layer transmits data (RLC SDU or RLC PDU) as complete data without segmenting a RLC SDU or when the transmission RLC layer transmits a first segment segmented from a RLC SDU, the transmission RLC layer may apply, in the case that a 18-bit RLC SN has been set in the RLC layer, a RLC header 2 h-10 including a RLC SN and a SI field (indicating the complete data, the first segment, a final segment, or a middle segment being neither the first segment nor the final segment) to the data and transmit the data. However, when the transmission RLC layer segments the RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 h-20 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

Hereinafter, a second embodiment for operations of the reception RLC layer proposed in the disclosure will be described in detail in regard of the RLC UM mode and the RLC AM mode.

In the second embodiment for the operations of the reception RLC layer according to an embodiment of the disclosure, the RLC layer may order data based on RLC SNs and transfer the data to an upper layer, when setting or activating of the in-order delivery function is indicated with an RRC message or a RLC control PDU by a gNB, although the RLC layer performs the out-of-order delivery function as a basic operation. Also, according to the above-described second embodiment, when the RLC layer applies the out-of-order delivery function, the RLC layer may immediately discard segments being out of the RLC reassembly window. However, when the in-order delivery function is set or activated, the RLC layer may attempt to reassemble, instead of immediately discarding the segments being out of the RLC reassembly window, the segments being out of the RLC reassembly window, transfer, when the segments are reassembled into complete RLC SDUs, the RLC SDUs to an upper layer in ascending order of RLC SNs, and discard segments that has failed to be reassembled into the complete RLC SDUs. Also, when the in-order delivery function is set or activated in the RLC layer, the RLC layer may order, instead of immediately reassembling a RLC SDU corresponding to a RLC SN when all bytes of the RLC SDU is received and transferring the RLC SDU to the upper layer, RLC SDUs in order of RLC SNs, then reassemble the RLC SDUs, and transfer the RLC SDUs to the upper layer in ascending order of the RLC SNs. When the out-of-order delivery function is applied to the RLC layer, the RLC layer may immediately reassemble an RLC SDU corresponding to an RLC SN when all bytes of the RLC SDU are received, and then transfer the RLC SDU to the upper layer.

First, variables that are used in a window operation of the reception RLC layer in the RLC UM mode may be defined as follows.

-   -   RX_Next_Reassembly: status variable indicating a lowest or         earliest RLC SN among data (RLC PDU or RLC SDU) considered to be         reassembled in the reception RLC layer.     -   RX_Timer_Trigger: status variable indicating an RLC SN following         an RLC SN triggering the RLC reassembly timer     -   RX_Next_Highest: status variable indicating an RLC SN following         a highest RLC SN among received data

Detailed operations of a 2-1 embodiment when the reception RLC layer according to an embodiment of the disclosure operates in the RLC UM mode may be as follows.

The reception RLC layer may operate, when receiving a UMD PDU from a lower layer, as follows.

-   -   When no RLC SN is included in a header of the received UMD PDU,     -   Remove an RLC header and transfer an RLC SDU to an upper layer.     -   When an RLC SN is included in the header of the received UMD PDU         and the RLC SN is equal to or greater than a         RX_Next_Highest−UM_Window_Size value and smaller than a         RX_Next_Reassembly value,     -   Discard the received UMD PDU.     -   When an RLC SN is included in the header of the received UMD PDU         and the RLC SN is smaller than the         RX_Next_Highest−UM_Window_Size value or equal to or greater than         the RX_Next_Reassembly value,     -   Store the received UMD PDU in a reception buffer.

The reception RLC layer may operate in regard of the UMD PDU stored in the buffer in the above-described procedure, as follows. That is, when a UMD PDU having an RLC SN of an x value is stored in the buffer, the reception RLC layer may operate as follows.

-   -   When all segments having the RLC SN of the x value are received,     -   When the in-order-delivery function is not set,     -   Reassemble all the segments having the RLC SN of the x value to         configure an RLC SDU, remove an RLC header, and then transfer         the resultant RLC SDU to an upper layer.     -   When the x value is equal to a RX_Next_Reassembly value,     -   Update a RX_Next_Reassembly variable to a RLC SN of first data         (or data having a lowest RLC SN among data having great RLC SNs)         among data neither yet reassembled nor transferred to an upper         layer, the RLC SN being greater than the current         RX_Next_Reassembly value.     -   Reassemble UMD PDUs having RLC SNs that are smaller than the         updated RX_Next_Reassembly value into RLC SDUs, remove RLC         headers, and transfer the reassembled RLC SDUs to the upper         layer in ascending order of the RLC SNs when the reassembled RLC         SDUs have never previously been transferred to the upper layer.     -   When the RLC SN of the x value is out of the RLC reassembly         window,     -   Update a RX_Next_Highest variable to x+1.     -   When the in-order delivery function is not set,     -   Discard all data corresponding to RLC SNs being out of the RLC         reassembly window.     -   Reassemble UMD PDUs having the RLC SNs being out of the RLC         reassembly window into RLC SDUs, remove RLC headers, and         transfer the reassembled RLC SDUs to the upper layer in         ascending order of the RLC SNs when the reassembled RLC SDUs         have never previously been transferred to the upper layer.     -   When the RX_Next_Reassembly value is out of the RLC reassembly         window,     -   Set the RX_Next_Reassembly variable to the RLC SN of the first         data (or data having the lowest RLC SN among the data having the         great RLC SNs) among the data neither yet reassembled nor         transferred to the upper layer, the RLC SN being equal to or         greater than the current RX_Next_Highest−UM_Window_Size value.     -   When the RLC reassembly timer is being driven,     -   When a RX_Timer_Trigger value is equal to or smaller than the         RX_Next_Reassumbly value,     -   Or, when the RX_Timer_Trigger value is out of the RLC reassembly         window and the RX_Timer_Trigger value is not equal to the         RX_Next_Highest value,     -   Or, when the RX_Next_Highest value is equal to a         RX_Next_Reassembly+1 value and there is no segment lost before a         final byte of all segments currently received for a RLC SDU         having the RX_Next_Reassembly value as a RLC SN,     -   Stop and initialize the RLC reassembly timer.     -   When the RLC reassembly timer is not driven (including a case in         which the RLC reassembly timer stops according to a procedure),     -   When the RX_Next_Highest value is greater than a         RX_Next_Reassumbly+1 value,     -   Or, when the RX_Next_Highest value is equal to the         RX_Next_Reassumbly+1 value and there is at least one segment         lost before a final byte of all segments currently received for         a RLC SDU having the RX_Next_Reassembly value as a RLC SN,     -   Start the RLC reassembly timer, and     -   Set the RX_Timer_Trigger variable to the RX_Next_Highest value.

When the RLC reassembly timer is expired in the above-described procedure, the reception RLC layer may operate as follows.

-   -   Update the RX_Next_Reassembly variable to an RLC SN of first         data (or data having a lowest RLC SN) not yet reassembled, the         RLC SN being equal to or greater than the RX_Timer_Trigger         value.     -   When the in-order delivery function is not set,     -   Discard all segments having RLC SNs that are smaller than the         updated RX_Next_Reassembly value.     -   Reassemble UMD PDUs having RLC SNs that are smaller than the         updated RX_Next_Reassembly value into RLC SDUs, remove RLC         headers, and transfer the reassembled RLC SDUs to the upper         layer in ascending order of the RLC SNs when the reassembled RLC         SDUs have never previously been transferred to the upper layer.     -   When the RX_Next_Highest value is greater than a         RX_Next_Reassumbly+1 value,     -   Or, when the RX_Next_Highest value is equal to the         RX_Next_Reassumbly+1 value and there is at least one segment         lost before a final byte of all segments currently received for         a RLC SDU having the RX_Next_Reassembly value as a RLC SN,     -   Start the RLC reassembly timer, and     -   Set the RX_Timer_Trigger variable to the RX_Next_Highest value.

FIGS. 2IA and 21B shows RLC header formats that may be applied in the 2-1 embodiment which are the operations of the reception RLC layer in which the RLC UM mode according to an embodiment of the disclosure is set.

Referring to FIG. 2I, in the 2-1 embodiment which are the operations of the reception RLC layer in which the RLC UM mode according to an embodiment of the disclosure is set, when the in-order delivery function is not set, the transmission RLC layer may apply, when transmitting data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU, a RLC header 2 i-05 to the data and transmit the data in the case that a 6-bit RLC SN has been set in the RLC layer. The RLC header 2 i-05 includes a SI field (indicating the complete data, a first segment, a final segment, or a middle segment being neither the first segment nor the final segment) without an RLC SN. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 i-10 including a RLC SN and a SI field to a first segment and transmit the first segment, and apply a RLC header 2 i-20 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to a middle or final segment and transmit the middle or final segment.

Also, when the in-order delivery function is not set, the transmission RLC layer may apply, when transmitting data (RLC SDU or RLC PDU) as complete data without separating the RLC SDU, the RLC header 2 i-05 including a SI field in the data without a RLC SN to the data and transmit the data, in the case in which a 12-bit RLC SN has been set in the RLC layer. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 i-15 including a RLC SN and a SI field in the first segment and transmit the first segment, and apply a RLC header 2 i-25 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

In the 2-1 embodiment which are the operations of the reception RLC layer in which the RLC UM mode according to an embodiment of the disclosure is set, when the in-order delivery function is set, the transmission RLC layer may apply, when transmitting data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU, a RLC header 2 i-10 including a RLC SN and a SI field (indicating the complete data, a first segment, a final segment, or a middle segment being neither the first segment nor the final segment) to the data and transmit the data, in the case that a 6-bit RLC SN has been set in the RLC layer, such that the reception RLC layer is capable of ordering the data. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply the RLC header including a RLC SN and a SI field to the first segment and transmit the first segment, and apply the RLC header 2 i-20 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

According to another method, in the 2-1 embodiment which are the operations of the reception RLC layer in which the RLC UM mode according to an embodiment of the disclosure is set, when the in-order delivery function is set, the transmission RLC layer may apply, when transmitting data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU, a RLC header 2 i-30 including a RLC SN to the data and transmit the data, in the case that a 6-bit RLC SN has been set in the RLC layer such that the reception RLC layer is capable of ordering the data. The RLC header 2 i-30 includes a 12-bit RLC SN and a SI field (indicating the complete data, a first segment, a final segment, or a middle segment being neither the first segment nor the final segment), a 1-bit I field, defined by the transmission RLC layer, indicating that the in-order delivery function is capable of being set or applied. That is, the transmission RLC layer may change a length of an RLC SN from 6 bits to 12 bits, and indicate the in-order delivery function with a 1-bit indicator. However, when the transmission RLC layer segments an RLC SDU and transmits segments, the transmission RLC layer may apply the RLC header 2 i-30 to a first segment and transmit the first segment. The RLC header includes an RLC SN, a SI field and a 1-bit I field, defined by the transmission RLC layer, including an indication representing that the in-order delivery function is capable of being set or applied. Also, the transmission RLC layer may apply an RLC header 2 i-35 to a middle or final segment and transmit the middle or final segment. The RLC header 2 i-35 includes a RLC SN, a SI field, a SO field (indicating a first byte segmented from a RLC SDU), and a 1-bit I field, defined by the transmission RLC layer, including an indication representing that the in-order delivery function may be set or applied.

Also, when the in-order delivery function is set, the transmission RLC layer may apply, when transmitting data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU, the RLC header 2 i-15 including a RLC SN and a SI field and transmit the data, in the case that a 12-bit RLC SN has been set in the RLC layer. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply the RLC header 2 i-15 including a RLC SN and a SI field to the first segment and transmit the first segment, and may apply the RLC header 2 i-25 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from a RLC SDU) to the middle or final segment and transmit the middle or final segment.

According to another method, when the in-order delivery function is set, the transmission RLC layer may apply, when transmitting data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU, the RLC header 2 i-30 to the data and transmit the data, in the case that a 12-bit RLC SN has been set in the RLC layer. The RLC header 2 i-30 includes an RLC SN, a SI field, and a 1-bit I field, defined by the transmission RLC layer, including an indication representing that the in-order delivery function is capable of being set or applied. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply the RLC header 2 i-30 including a RLC SN, a SI field, and a 1-bit I field, defined by the transmission RLC layer, including an indication representing that the in-order delivery function is capable of being set or applied to the first segment and transmit the first segment, and apply the RLC header 2 i-35 including a RLC SN, a SI field, a SO field (indicating a first byte segmented from a RLC SDU), and a 1-bit I field, defined by the transmission RLC layer, including an indication representing that the in-order delivery function is capable of being set or applied to the middle or final segment and transmit the middle or final segment.

According to an embodiment of the disclosure, the transmission RLC layer may define a I field in a header format of 2 i-05, set the I field to 0 like a header format of 2 i-40, and then apply the header format of 2 i-40 to a RLC SDU not segmented in the case that the in-order delivery function is not set.

Also, when the in-order delivery function is set or activated, the transmission RLC layer may configure an RLC header format including the I field according to an embodiment of the disclosure and transmit data including the RLC header format. When the in-order delivery function is set or activated, the reception RLC layer may check the I field to identify an RLC SN, and order the received data. The reception RLC layer may immediately reassemble a RLC PDU of which an I field value is 0 without applying the in-order delivery function to the RLC PDU although the in-order delivery function is set, remove a RLC header, and then transfer the resultant RLC PDU to an upper layer (because the RLC PDU is data transmitted before the in-order delivery function is set).

Detailed operations of a 2-2 embodiment when the reception RLC layer according to an embodiment of the disclosure operates in the RLC AM mode may be as follows. In the RLC AM mode, the RLC layer may implement an ARQ function, receive an RLC status report (indicating successful data transfer with an RLC SN) from a receiving terminal, and retransmit data not successfully transmitted, thereby preventing data loss.

First, in the RLC AM mode, variables that are used for a window operation of the reception RLC layer may be defined as follows.

-   -   RX_Next: status variable indicating an RLC SN following a         highest or final RLC SN among completely received data which is         capable of being reassembled     -   RX_Next_Status_Trigger: status variable indicating an RLC SN         following an RLC SN triggering the RLC assembly timer     -   RX_Next_Highest: status variable indicating an RLC SN following         a highest RLC SN among received data     -   RX_Next_Highest_Status: status variable indicating a highest RLC         SN (for example, an RLC SN following a highest RLC SN         successfully received) that may be indicated as ACK_SN in an RLC         status report when the RLC status report is configured

When the reception RLC layer receives an AMD PDU including y to z bytes of an RLC SDU having an RLC SN of a x value from a lower layer, the reception RLC layer may operate as follows.

-   -   When the RLC SN of the x value of the received AMD PDU is out of         an RLC reception window,     -   When an AMD PDU including y to z bytes of an RLC SDU having the         RLC SN of the x value has been previously received,     -   Discard the received AMD PDU.     -   Otherwise,     -   Store the received AMD PDU in the reception buffer.     -   When a portion or segment included in the AMD PDU has been         previously received,     -   Consider that the portion or segment included in the AMD PDU has         been received in duplicate and discard the portion or segment.

The reception RLC layer may operate in regard of the AMD PDU stored in the buffer in the above-described procedure, as follows. That is, when the AMD PDU having the RLC SN of the x value is stored in the buffer, the reception RLC layer may operate as follows.

-   -   When the x value of the RLC SN is equal to or greater than a         RX_Next_Highest value,     -   Update the RX_Next_Highest value to x+1     -   When all bytes of an RLC SDU corresponding to the x value of the         RLC SN are received,     -   When the in-order delivery function is not set,     -   Reassemble data or segments corresponding to the x value of the         RLC SN to configure an RLC SDU, remove an RLC header, and then         transfer the reassembled RLC SDU to an upper layer,     -   When the x value is equal to a RX_Next_Reassembly value,     -   Update a RX_Highest_Status variable to an RLC SN of first data         (having a lowest RLC SN) which is greater than the         RX_Highest_Status value and of which all bytes are not yet         completely received.     -   When the x value is equal to a RX_Next value,     -   update a RX_Next variable to an RLC SN of first data (having a         lowest RLC SN) which is greater than the RX_Next value and of         which all bytes are not yet completely received.     -   Reassemble AMD PDUs having RLC SNs that are smaller than the         updated RX_Next value into RLC SDUs, remove RLC headers, and         transfer the reassembled RLC SDUs to the upper layer in         ascending order of the RLC SNs when the reassembled RLC SDUs         have never previously been transferred to the upper layer.

When the RLC reassembly timer is being driven,

-   -   When a RX_Next_Status_Trigger value is equal to the RX_Next         value,     -   Or, when the RX_Next_Status_Trigger value is equal to a         RX_Next+1 value and there is no segment lost before a final byte         of all segments currently received for a RLC SDU having the         RX_Next value as a RLC SN,     -   Or, when the RX_Next_Status_Trigger value is out of an RLC         reception window and the RX_Next_Status_Trigger value is not         equal to a RX_Next+AM_Window_Size value,     -   Stop and initialize the RLC reassembly timer.     -   When the RLC reassembly time is not driven (including a case in         which the RLC reassembly timer stops according to a procedure),     -   When the RX_Next_Highest value is greater than the RX_Next+1         value,     -   Or, when the RX_Next_Highest value is equal to the RX_Next+1         value and there is at least one segment lost before a final byte         of all segments currently received for a RLC SDU having the         RX_Next value as a RLC SN,     -   Start the RLC reassembly timer, and     -   Set the RX_Next_Status_Trigger variable to the RX_Next_Highest         value.

When the RLC reassembly timer is expired in the above-described procedure, the reception RLC layer may operate as follows.

-   -   Update the RX_Highest_Status variable to an RLC SN of first data         (data having a lowest RLC SN) not yet completely received, the         RLC SN being equal to or greater than the RX_Next_Status_Trigger         value.     -   When the RX_Next_Highest value is greater than the RX_Next+1         value,     -   Or, when the RX_Next_Highest value is equal to the RX_Next+1         value and there is at least one segment lost before a final byte         of all segments currently received for a RLC SDU having the         RX_Next value as a RLC SN,     -   Start the RLC reassembly timer, and     -   Set the RX_Timer_Status_Trigger variable to the RX_Next_Highest         value.

FIG. 2J shows RLC header formats that may be applied in the 2-2 embodiment which are operations of the reception RLC layer in which the RLC AM mode according to an embodiment of the disclosure is set.

Referring to FIG. 2J, in the 2-2 embodiment which are operations of the reception RLC layer in which the RLC AM mode according to an embodiment of the disclosure is set, when the transmission RLC layer transmits data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU or when the transmission RLC layer transmits a first segment segmented from the RLC SDU, the transmission RLC layer may apply, in the case that a 12-bit RLC SN has been set in the RLC layer, a RLC header 2 j-05 including a RLC SN and a SI field (indicating the complete data, the first segment, a final segment, or a middle segment being neither the first segment nor the final segment) to the data and transmit the data. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 j-15 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

Also, in the 2-2 embodiment which are the operations of the reception RLC layer in which the RLC AM mode according to an embodiment of the disclosure is set, when the transmission RLC layer transmits data (RLC SDU or RLC PDU) as complete data without segmenting the RLC SDU or when the transmission RLC layer transmits a first segment segmented from the RLC SDU, the transmission RLC layer may apply, in the case that a 18-bit RLC SN has been set in the RLC layer, a RLC header 2 j-10 including a RLC SN and a SI field (indicating the complete data, the first segment, a final segment, or a middle segment being neither the first segment nor the final segment) to the data and transmit the data. However, when the transmission RLC layer segments a RLC SDU and transmits segments, the transmission RLC layer may apply a RLC header 2 j-20 including a RLC SN, a SI field, and a SO field (indicating a first byte segmented from the RLC SDU) to the middle or final segment and transmit the middle or final segment.

In the 2-2 embodiment which are the operations of the reception RLC layer in which the RLC AM mode according to an embodiment of the disclosure is set, because a RLC SN is included in each data unlike the RLC UM mode, the same RLC headers shown in FIG. 2J may be applied to both a case that the in-order delivery function is set and a case that the in-order delivery function is not set, according to lengths of RLC SNs, as described above. That is, in the RLC AM mode, the same RLC header format may be applied and used regardless of whether or not the in-order delivery function is set, unlike the RLC UM mode. According to another method, the transmission RLC layer may define a 1-bit indicator like the RLC UM mode to indicate that the in-order delivery function is set, and define and use an RLC header format.

FIG. 2K shows operations of a reception RLC layer of a UE according to an embodiment of the disclosure.

Referring to FIG. 2K, the reception RLC layer (2 k-01) of the UE may receive data from a lower layer, in operation 2 k-05.

The UE may determine whether the in-order delivery function is set, based on the received data, in operation 2 k-10.

When the in-order delivery function is set, the UE may apply the above-described 2-1 embodiment to the RLC UM mode in consideration of a RLC header, and apply the above-described 2-2 embodiment to the RLC AM mode in consideration of a RLC header, in operation 2 k-20. For example, the UE may activate the in-order delivery function to order and process the received data and transfer the resultant data to an upper layer.

When the in-order delivery function is not set, the UE may apply the 1-1 embodiment or the 2-1 embodiment for the reception RLC layer to the RLC UM mode in consideration of a RLC header, and apply the 1-2 embodiment or the 2-2 embodiment for the reception RLC layer to the RLC AM mode in consideration of a RLC header, in operation 2 k-15. For example, the UE may process the received data and transfer the processed data to the upper layer according to the out-of-order delivery function.

FIG. 2L shows a structure of a UE or a wireless node to which an embodiment of the disclosure may be applied.

Referring to FIG. 2L, the UE may include an RF processor 2 l-10, a baseband processor 2 l-20, a storage device 2 l-30, and a controller 2 l-40. However, an internal structure of the UE is not limited to the embodiment shown in FIG. 2L, and the UE may include more or less components than those shown in FIG. 2L. The RF processor 2 l-10 may perform a function for transmitting and receiving signals through a wireless channel, such as band conversion, amplification, etc. of signals. That is, the RF processor 2 l-10 may up-convert a baseband signal provided from the baseband processor 2 l-20 into a RF band signal, transmit the RF band signal through an antenna, and down-convert an RF band signal received through the antenna into a baseband signal. For example, the RF processor 2 l-10 may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc. In FIG. 2I, a single antenna is shown, however, the UE may include a plurality of antennas. Also, the RF processor 2 l-10 may include a plurality of RF chains. Furthermore, the RF processor 2 l-10 may perform beamforming. For beamforming, the RF processor 2 l-10 may adjust phases and magnitudes of signals that are transmitted/received through the plurality of antennas or antenna elements. Also, the RF processor 2 l-10 may perform MIMO, and when MIMO is performed, the RF processor 2 l-10 may receive several layers. The RF processor 2 l-10 may perform sweeping of a received beam by appropriately setting the plurality of antennas or antenna elements according to a control of the controller 2 l-40, or may adjust a direction and width of a received beam such that the received beam coordinates with a beam that is to be transmitted.

The baseband processor 2 l-20 may perform a function of conversion between baseband signals and bit streams according to a physical layer specification of a system. For example, upon data transmission, the baseband processor 2 l-20 may encipher and modulate a transmission bit stream to generate complex symbols. Also, upon data reception, the baseband processor 2 l-20 may demodulate and decipher a baseband signal provided from the RF processor 2 l-10 to restore a reception bit stream. For example, according to an OFDM method, upon data transmission, the baseband processor 2 l-20 may encipher and modulate a transmission bit stream to generate complex symbols, map the complex symbols to subcarriers, and then construct OFDM symbols through IFFT and CP insertion. Also, upon data reception, the baseband processor 2 l-20 may segment a baseband signal provided from the RF processor 2 l-10 in units of OFDM symbols, restore signals mapped to subcarriers through FFT, and then restore a reception bit stream through demodulation and deciphering.

The baseband processor 2 l-20 and the RF processor 2 l-10 may transmit and receive signals as described above. Accordingly, the baseband processor 2 l-20 and the RF processor 2 l-10 are also referred to as a transmitter, a receiver, a transceiver, or a communicator. Furthermore, at least one of the baseband processor 2 l-20 or the RF processor 2 l-10 may include a plurality of communication modules to support a plurality of different radio access technologies. Also, at least one of the baseband processor 2 l-20 or the RF processor 2 l-10 may include different communication modules to process signals of different frequency bands. For example, the different radio access technologies may include a LET network, a NR network, etc. Also, the different frequency bands may include a SHF (e.g., 2.1 GHz and 1 GHz) band and a mm wave (e.g., 60 GHz) band. The UE may transmit/receive a signal to/from a gNB by using the baseband processor 2 l-20 and the RF processor 2 l-10, wherein the signal may include control information and data.

The storage device 2 l-30 may store data, such as basic programs, application programs, and configuration information, for operations of the UE. The storage device 2 l-30 may provide the stored data according to a request from the controller 2 l-40. The storage device 2 l-30 may be configured with storage media, such as ROM, RAM, a hard disc, CD-ROM, DVD, and the like, or a combination of such storage media. Also, the storage device 2 l-30 may be configured with a plurality of memories.

The controller 2 l-40 may control overall operations of the UE. For example, the controller 2 l-40 may transmit and receive signals through the baseband processor 2 l-20 and the RF processor 2 l-10. Also, the controller 2 l-40 may write data in the storage device 2 l-30 or read data from the storage device 2 l-30. To write or read data, the controller 2 l-40 may include at least one processor. For example, the controller 2 l-40 may include a communication processor CP for performing control for communication, and an application processor AP for controlling an upper layer of an application program, etc. Also, the controller 2 l-40 may include multiple connection processor 2 l-42 for processing multiple connections. Also, the controller 2 l-40 may control the UE to perform a method for processing communication data described above. Also, at least one component of the UE may be implemented as a single chip.

FIG. 2M shows a block configuration of a TRP or a wireless node in a wireless communication system according to an embodiment of the disclosure.

Referring to FIG. 2M, a gNB (also, referred to as a TRP or a wireless node) may include a RF processor 2 m-10, a baseband processor 2 m-20, a backhaul communicator 2 m-30, a storage device 2 m-40, and a controller 2 m-50. However, an internal structure of the gNB is not limited to the embodiment shown in FIG. 2M, and the gNB may include more or less components than those shown in FIG. 2M.

The RF processor 2 m-10 may perform a function for transmitting and receiving signals through a wireless channel, such as band conversion, amplification, etc. of signals. That is, the RF processor 2 m-10 may up-convert a baseband signal provided from the baseband processor 2 m-20 into a RF band signal, transmit the RF band signal through an antenna, and down-convert an RF band signal received through the antenna into a baseband signal. For example, the RF processor 2 m-10 may include a transmission filter, a reception filter, an amplifier, a mixer, an oscillator, a DAC, an ADC, etc. In FIG. 2M, a single antenna is shown, however, the gNB may include a plurality of antennas. Also, the RF processor 2 m-10 may include a plurality of RF chains. Furthermore, the RF processor 2 m-10 may perform beamforming. For beamforming, the RF processor 2 m-10 may adjust phases and magnitudes of signals that are transmitted/received through the plurality of antennas or antenna elements. The RF processor 2 m-10 may perform downlink MIMO by transmitting one or more layers.

The baseband processor 2 m-20 may perform a function of conversion between baseband signals and bit streams according to a physical layer specification of first radio access technology. For example, upon data transmission, the baseband processor 2 m-20 may encipher and modulate a transmission bit stream to generate complex symbols. Also, upon data reception, the baseband processor 2 m-20 may demodulate and decipher a baseband signal provided from the RF processor 2 m-10 to restore a reception bit stream. For example, according to the OFDM method, upon data transmission, the baseband processor 2 m-20 may encipher and modulate a transmission bit stream to generate complex symbols, map the complex symbols to subcarriers, and then construct OFDM symbols through IFFT and CP insertion. Also, upon data reception, the baseband processor 2 m-20 may segment a baseband signal provided from the RF processor 2 m-10 in units of OFDM symbols, restore signals mapped to subcarriers through FFT, and then restore a reception bit stream through demodulation and deciphering. The baseband processor 2 m-20 and the RF processor 2 m-10 may transmit and receive signals as described above. Accordingly, the baseband processor 2 m-20 and the RF processor 2 m-10 are also referred to as a transmitter, a receiver, a transceiver, or a wireless communicator. The gNB may transmit/receive a signal to/from the UE by using the baseband processor 2 m-20 and the RF processor 2 m-10, wherein the signal may include control information and data.

The communicator 2 m-30 may provide an interface for communicating with other nodes in a network.

The storage device 2 m-40 may store data, such as basic programs, application programs, and configuration information, for operations of the gNB. Particularly, the storage device 2 m-40 may store information about a bearer assigned to a UE connected to the gNB, a measurement result reported from the connected UE, etc. Also, the storage device 2 m-40 may store information that is used as a determination criterion about whether to provide multiple connections to the UE or whether to release multiple connections to the UE. Also, the storage device 2 m-40 may provide the stored data according to a request from the controller 2 m-50. The storage device 2 m-40 may be configured with storage media, such as ROM, RAM, a hard disc, CD-ROM, DVD, and the like, or a combination of such storage media. Also, the storage device 2 m-40 may be configured with a plurality of memories.

The controller 2 m-50 may control overall operations of the gNB. For example, the controller 2 m-50 may transmit and receive signals through the baseband processor 2 m-20 and the RF processor 2 m-10 or through the backhaul communicator 2 m-30. Also, the controller 2 m-50 may write data in the storage device 2 m-40 or read data from the storage device 2 m-40. To write or read data, the controller 2 m-50 may include at least one processor. Also, the controller 2 m-50 may include multiple connection processor 2 m-52 for processing multiple connections. For example, the controller 2 m-50 may control the gNB such that the UE may perform a method for performing communication data as described above. Also, at least one component in the gNB may be implemented as a single chip.

The methods according to the embodiments of the disclosure described in claims or specification thereof may be implemented in hardware, software, or a combination of hardware and software.

When the methods are implemented in software, a computer-readable storage medium storing at least one program (software module) may be provided. The at least one program stored in the computer-readable storage medium may be configured for execution by at least one processor within an electronic device. The at least one program may include instructions that cause the electronic device to execute the methods according to the embodiments of the disclosure described in the claims or specification thereof.

The program (software module or software) may be stored in RAM, a non-volatile memory including a flash memory, ROM, an electrically erasable programmable ROM (EEPROM), a magnetic disc storage device, CD-ROM, DVDs or other types of optical storage devices, and a magnetic cassette. Alternatively, the program may be stored in a memory that is configured as a combination of some or all of the memories. A plurality of such memories may be included.

Furthermore, the program may be stored in an attachable storage device that may be accessed through communication networks such as the Internet, Intranet, a local area network (LAN), a wide LAN (WLAN), and a storage area network (SAN) or a communication network configured in a combination thereof. The storage device may access a device performing the embodiments of the disclosure through an external port. Further, a separate storage device on the communication network may also access the device performing the embodiments of the disclosure.

In the embodiments of the disclosure, a component included in the disclosure is expressed in a singular or plural form depending on the described embodiments of the disclosure. However, singular or plural expressions are selected to be suitable for the presented situations for convenience, and the disclosure is not limited to the singular or plural form. A component expressed in a plural form may be configured as a single component, or a component expressed in a singular form may be configured as a plurality of components.

The disclosed embodiments provide an apparatus and method capable of effectively providing a service to a mobile communication system.

Meanwhile, the embodiments of the disclosure disclosed in the present specification and the drawings are specific examples intended to easily describe the technical content of the disclosure and help understanding the disclosure, not intended to limit the scope of the disclosure. That is, it is apparent to those skilled in the art that other modified examples based on the technical spirit of the disclosure may be executed. Also, the embodiments may be combined and used as needed. For example, some parts of the embodiments of the disclosure may be combined to operate the gNB and UE. Also, the embodiments of the disclosure may be applied to other communication systems, and other modified examples based on the technical concept of the embodiments may also be executed.

Although the present disclosure has been described with various embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims. 

What is claimed is:
 1. A method performed by a terminal in a wireless communication system, the method comprising: receiving, from a base station, a radio resource control (RRC) reconfiguration message comprising configuration information for conditional handover; identifying a cell for conditional handover based on the configuration information for conditional handover; and triggering a packet data convergence protocol (PDCP) status report for a radio bearer according to an execution of a conditional handover for the identified cell for conditional handover.
 2. The method of claim 1, further comprising reestablishing a PDCP entity corresponding to the RRC reconfiguration message, wherein the PDCP status report is triggered in response to the reestablishment of the PDCP entity.
 3. The method of claim 1, wherein the RRC reconfiguration message comprises PDCP configuration information, and wherein the PDCP status report is triggered in case that the PDCP configuration information comprises information indicating that the PDCP status report is required.
 4. The method of claim 1, wherein the radio bearer is an acknowledged mode (AM) data radio bearer (DRB).
 5. The method of claim 1, wherein the configuration information for conditional handover comprises information of at least one candidate cell for conditional handover and information of conditions for conditional handover.
 6. The method of claim 5, wherein the identifying of the cell for conditional handover comprises identifying the cell for conditional handover from among the at least one candidate cell based on the information of conditions for conditional handover.
 7. The method of claim 1, further comprising transmitting the PDCP status report to the identified cell for conditional handover.
 8. A terminal in a wireless communication system, the terminal comprising: a transceiver; and at least one controller configured to: receive, from a base station, a radio resource control (RRC) reconfiguration message comprising configuration information for conditional handover, identify a cell for conditional handover based on the configuration information for conditional handover, and trigger a packet data convergence protocol (PDCP) status report for a radio bearer according to an execution of a conditional handover for the identified cell for conditional handover.
 9. The terminal of claim 8, wherein the at least one controller is further configured to reestablish a PDCP entity corresponding to the RRC reconfiguration message, and wherein the PDCP status report is triggered in response to the reestablishment of the PDCP entity.
 10. The terminal of claim 8, wherein the RRC reconfiguration message comprises PDCP configuration information, and wherein the PDCP status report is triggered in case that the PDCP configuration information comprises information indicating that the PDCP status report is required.
 11. The terminal of claim 8, wherein the radio bearer is an acknowledged mode (AM) data radio bearer (DRB).
 12. The terminal of claim 8, wherein the configuration information for conditional handover comprises information of at least one candidate cell for conditional handover and information of conditions for conditional handover.
 13. The terminal of claim 12, wherein at least one controller is further configured to identify the cell for conditional handover from among the at least one candidate cell based on the information of conditions for conditional handover.
 14. The terminal of claim 8, wherein at least one controller is further configured to transmit the PDCP status report to the identified cell for conditional handover. 